Register marks

Abstract: A register mark and a system for bringing a pair of register marks into alignment is disclosed. A first two-dimensional register mark (Fig. 1) comprises a plurality of dots of a first frequency, and a second two-dimensional register mark (Fig. 2) comprises a plurality of dots of a second, generally higher frequency. When the first and second register marks are overlaid, an interference pattern resulting from the difference in frequency is observed. The first and second register marks are in alignment when the interference pattern produces a maximum bright spot in the center of the register mark. A small relative movement of the first and second register marks results in a larger relative movement of center of the bright spot. The position of the bright spot indicates which direction and how much to move the second register mark in order to achieve image registration. It is preferred that the frequency of dots in each register mark is warped with distance from the center of the mark. The frequency of the dots of the second register mark is also warped with distance from the center, but overall has a generally higher frequency of dots as compared to the first register mark. In the resulting interference pattern the bright spot occurs unambiguously when the two register marks are in alignment. At misalignments of more than one dot width, the bright spot breaks up so that the overlaid register marks may serve as quick visual check of proper alignment. The register marks may be in a single dimension along a line.

Description: The present invention is concerned with field of register marks used for adjusting the relative position of two or more objects in order to bring the objects into alignment. The invention relates to a register mark arrangement and to a method and apparatus for aligning two objects using register marks. The invention further relates to monitoring the alignment of two objects such as layers using register marks. Particular, though not exclusive, applications of the invention are to the alignment of semiconductor masks and color printing. In many manufacturing processes, it is often necessary to align two objects, such as two layers in which one layer will overlie the other. For example, in semiconductor manufacturing, successive masks which define the semiconductor structure are placed over the semiconductor substrate at various stages in the manufacturing process. Each successive mask must precisely align with the position of a previous mask on the semiconductor substrate. As another example, in color printing, an original color image is scanned to separate the original image into several component colors, each of which component color will be separately printed in alignment with the layers above and below. Therefore, the color printing plates must be manufactured so that each of the separate color images are printed in precise alignment or registration with each other. Specifically, an original input image is processed to produce four screened images: one for each of the printing inks of cyan, magenta, yellow and black. The four screened images are then used to burn printing plates needed in a four color printing reproduction process. In burning the plate, the image negative is affixed to a transparent acetate sheet with holes punched. The acetate sheet is then placed on the plate burning machine, which has metal tabs that fit into the holes in the acetate sheet. Each of the acetate sheets must have negatives at exactly the same position relative to the holes in order to insure that the plates have images in the same place. Accurate placement of the negatives on the acetate sheets is even more important if each plate is to be burned with more than one negative each. To accurately place the negative on the acetate sheet, register marks are used. The first negative, typically corresponding to the black image plane, is placed on a light table that has metal tabs to fit the holes in the acetate sheet. The location of the first negative on the first acetate sheet is not too critical; a typical tolerance is on the order of 0,13 mm (50 mils). The first negative placement determines the general location of the color image on the printed page. The next step is to put another acetate sheet over the first sheet, and place a second negative, say the negative corresponding to the magenta image plane on the second acetate sheet in precise relation to the first negative on the first acetate sheet. To facilitate alignment, image negatives have register marks which are not part of the image and are later removed. The operator examines the register marks, usually with the aid of a magnifier. By close examination of the register marks, the operator determines the position of the second negative on the second acetate sheet for which the two register marks are exactly overlaid. The second acetate sheet carrying the aligned second image negative (magenta) is then removed. The process is repeated for a third acetate sheet and a third negative, say for cyan, and again for yellow. The acetate sheets are then used as indicated above to burn printing plates. The quality of color printing is dependent upon the accuracy of alignment of the four color images in the overall plate burning and printing process. Automated processes for the automatic alignment of register marks are also known. The composite image of the two register marks is captured in a memory, the alignment error is measured, and one plate is moved in a direction so as to reduce the measured error. When the measured error is minimized, the register marks are aligned. There are many known prior art register marks used to align two overlying transparent layers. One type of conventional register mark consists of crosshairs, i.e. a plus sign. This form of register mark has two disadvantages. First, a crosshairs register mark requires close examination, usually with the aid of a magnifier, to determine exact alignment. Second, when the two planes are not in good register, it is very difficult to determine from the non-aligned crosshair composite image which direction to move the top plane. Generally, trial and error movements indicate to the observer which direction to move the top plane. The need for close inspection and trial movements makes automated adjustment complex and expensive, in both the optical system and in the mechanical control system. It is known to provide register marks which act as diffraction gratings for light, such as a laser beam, impinging on them. The alignment of the register marks is determined by monitoring the respective patterns of the diffracted light. An example of this technique is disclosed in "Interferometric Method of Checking the Overlay Accuracy in Photolithographic Exposure Processes, IBM Technical Disclosure Bulletin, Vol. 32, No. 10B, March 1990. The arrangement there described monitors alignment along only one axis. Another interferometric method based on diffraction is disclosed in U.S. patent 4 704 033 to Fay et al. GB-A-2 183 364 discloses a register mark arrangement of the kind comprising: a first register mark including a plurality of elements, the frequency of the spacial arrangement of which changes across the first register mark, and a second register mark including a second plurality of elements, the frequency of the spacial arrangement of which changes across the second register mark. More specifically GB-A-2 183 364 discloses an alignment method involving detection of the interference (Moire) pattern between the two register marks when they are overlaid. Furthermore the marks are overlaid such that one is a mirror image of the other. The two register marks are identical and thus their elements have the same spacial frequency variation. This variation is in a single sense, that is from higher to lower along the axis of alignment. This is a chirp grating. The elements themselves are stripes transverse to and angled with respect to the axis of alignment. The interference pattern at the point of alignment is a straight band extending along this axis. Misalignment of the axis distorts the straight band by introducing a curvature. The principles of this technique are used in an adaptation for detection of alignment along two axes. Correct alignment is indicated by a cross having straight sides. Curvature of the sides results from misalignment. The present invention provides a register mark arrangement of the kind set forth above which is characterized in that: the elements of said first and second pluralities of elements comprise dots, the spacial frequency of each of said first and second pluralities of dots varies in the same sense with distance from the centre of said first and second register marks respectively, the spacial frequency of the dots of said second register mark is greater than the spacial frequency of the dots of said first register mark at corresponding areas of the first and second register marks, and said first and second pluralities of dots being arranged and said spacial frequencies chosen to produce an interference pattern including a bright spot at the centre of said register marks, when said register marks are disposed in overlaid relation, which is indicative of the alignment of said first and second register marks. The variation of spacial frequency away from the centres of the register marks is also referred to as frequency warping. The preferred register mark arrangement is a two-dimensional array of dots, the respective spacial frequencies of each plurality decreasing with radial distance from the respective centres of the register marks. The bright spot in the interference pattern produced in the register mark arrangement according to the invention occurs unambiguously when the two register marks are aligned. Furthermore at small displacements (explained later) from the aligned position, the bright spot moves and the detection of the position of the peak of the bright spot with respect to the centre of one register mark is usable to calculate an error which is a measure of the misalignment in both direction and amount and which may be used to control relative movement between the two register marks to bring them into alignment. By selecting the difference of the spacial frequencies of the dots of the two register marks, the displacement of the bright spot may be magnified with respect to the displacement of the register marks from alignment. A small relative movement of the first and second register marks results in a larger relative movement of the bright spot. The invention also provides a method of monitoring the alignment of and aligning first and second objects which carry first and second register marks respectively for locating in overlaid relationship to determine the alignment of the first and second objects as described in the claims 11 and 19. The first and second register marks provide a register mark arrangement in accordance with the invention. The invention further provides apparatus for monitoring the alignment of first and second objects carrying first and second register marks respectively, which register marks are located in overlaid relation; and apparatus for aligning such first and second objects as described in claims 21 and 22. In each case the first and second register marks provide a register mark arrangement in accordance with the invention. Applications of the method and apparatus of the invention are in aligning a semiconductor substrate and a mask or aligning two masks in the production of integrated circuits; and in colour printing, for example the production of printing plates from colour negatives relating to an image. Register marks having the characteristics defined above in accordance with the present invention may also be printed alongside the color image to check the final alignment of the overall printing process, or to monitor the operation of a high speed printing press. Furthermore, such register marks may be used in projection alignment systems where one register mark is printed or otherwise fixed onto a surface, and the other register mark is projected onto the first register mark, as for example in the manufacture of semiconductor integrated circuits. In general, as used herein, the term "print" shall mean either the deposition or removal of material so as to form an image on the surface of an object. The invention and its practice will be more fully described with reference to the accompanying drawings, in which: Figure 1 is an illustration of a bottom register mark for a register mark arrangement in accordance with the present invention; Figure 2 is an illustration of a complementary top register mark for the register mark arrangement; Figure 3 is an illustration of the pair of top and bottom register marks of Figs. 1 and 2 shown with the register marks in alignment; Figure 4 is an illustration of the same pair of top and bottom register marks shown with the register marks out of alignment by 3 pixels to the right and 2 pixels down; Figure 5 is an illustration of the same pair of top and bottom register marks shown with the register marks out of alignment by 13 pixels to the right and 5 pixels down; Figures 6A-6K illustrates successive superimpositions of a pair of top and bottom register marks, Figs. 6A showing the pair in alignment and each successive figure 6B-6K shown with top and bottom register marks out of alignment by an additional 2 pixels to the right; Figure 7 is a block diagram of an automatic alignment system used in conjunction with a pair of register marks in accordance with the present invention; and Figure 8 is a flow chart for the computer program in the CPU of figure 7. A first register mark 10 of a register mark arrangement in accordance with the present invention is shown in figure 1, as comprising a plurality of dots of a first frequency. At least one horizontal line of dots includes dots 14, 16 and 18. The frequency at the center of the horizontal line is greatest at dot 16 near the center of the horizontal line, while the frequency of dots 14 and 18 decreases generally at distances away from the center. Similarly, register mark 10 includes at least one vertical line of dots such as dots 20, 22 and 24. The frequency at the center of the vertical line is greatest at dot 22 near the center of the horizontal line, while the frequency of dots 20 and 24 decreases generally at distances away from the center. Figure 2 is a second register mark 12 of the register mark arrangement comprising a plurality of dots. The arrangement of the second register mark is similar to the first register mark, but the entire pattern is of a generally higher frequency. For example, register mark 12 may have 5% higher frequency at all points, which results in a slightly smaller size for the second register mark as compared to the first register mark. The use of these register marks as a pair will now be described. It is considered that these marks and others to be described are individually novel, as is the routine for generating them described below. Figure 3 shows the first and second register marks in register. When the first and second register marks are in alignment, an interference pattern, produced by the sum and difference of the frequencies of the first and second register marks produces a bright spot in the center of the register mark. The bright spot results because the dots of the register mark tend to be coincident near the center, minimizing black while maximizing white area. Due to the frequency difference, the dots at a distance from the center tend to be more and more non-coincident, resulting in more and more black area. The net effect is a bright spot with a peak at coordinates defined by column 30 and row 32. As the first and second register marks are moved out of alignment, the center of the bright spot moves away from the center in a direction so as to indicate the direction of the misalignment. Furthermore, if one mark is moved a small amount, the bright spot moves a much larger amount. The ratio is inversely proportional to the difference between the top and bottom register mark frequencies. By using a very small frequency difference between the two register marks, a magnification of 10 is easy to achieve. An example of an out of register condition is illustrated in figure 4 in which the top register mark is moved 3 pixels to the right and 2 pixels down. The center of the bright spot has moved to a coordinate location defined by column 34 and row 36, a much greater movement than 3 pixels right and 2 pixels down would indicate. As the first and second register marks-are moved even further out of alignment, by more than one dot width, the pattern becomes incoherent, i.e. there is no clearly defined bright spot. The latter situation is illustrated in figure 5 in which the top register mark is moved 13 pixels to the right and 5 pixels down. The patterns resulting from the use of pairs of register marks in accordance with the present invention are also known as moire patterns, which result from small differences in screen rulings and screen angles in the printing of color images. Although dots shown in register marks 10 and 12 are of a generally rectangular shape, other dot shapes will also work. If, instead of frequency warping, one register mark has a periodic pattern of a constant single frequency, and the second register mark has a periodic pattern of a constant single, slightly higher frequency, the constant frequency pattern will produce an appropriate interference pattern and bright spot to indicate alignment. However, in the case of a constant frequency pattern, as the bright spot moves quickly to one side, another bright spot will move in from the other side. When the top and bottom register marks are offset by exactly one dot cycle, the bright spot appears exactly the same as when the top and bottom register marks are in perfect register. Due to the above described ambiguity, a constant frequency pattern cannot be used as a quick visual check of proper alignment, i.e. as a go/no go indicator. The warping of the dot pattern frequency with distance from the center of the register mark, avoids the ambiguity which could result from constant frequency register marks offset by one dot cycle. An illustration of the interference patterns resulting from different relative horizontal positions of the top and bottom register marks is shown in figures 6A through 6K. In each successive figure, the top register mark is moved to the right, out of alignment by an additional 2 pixels as compared to the prior figure. The bright spot indicating alignment in figure 6A, moves further to the right in figures 6B, 6C, 6D and 6E, until substantially disappearing as shown in figure 6F. Figure 6F also shows another bright spot moving in from the other side. However, as shown in figures 6G, 6H, 6I, 6J and 6K the new bright spot breaks up before it moves to the center of the register mark. It is noted that two pairs of one dimensional register marks may also be used in lieu of the single pair of two dimensional register marks shown. In the latter case, one pair of register marks may provide a first bright spot in the form of a bright bar (instead of a circle) to indicate horizontal registration, while the other pair of register marks provides a second bright spot in the form of a bright bar to indicate vertical registration. A particular advantage of the register mark of the present invention is that the position of the bright spot indicates which direction and how much to move the register mark in order to achieve image registration, a feature particularly useful in automatic alignment systems. Figure 7 is a block diagram of a system for automatically aligning two layers 50 and 56 using register marks 52 and 54 of the present invention. The system comprises an image capture apparatus 40, which may be a video camera or a line of photosensors, an image memory 42, a central processing unit (CPU) 44, and actuators 58 and 60 for moving layer 50 relative to layer 56. In operation, layers 50 and 56 are placed in approximate alignment with register mark 52 overlying register mark 54. Camera 40 captures an image of the two register marks for storage in an image memory 42. CPU 44, responsive to the image memory 42 provides a vertical actuation signal on conductor 46 to vertical actuator 58, and a horizontal actuation signal on conductor 48 to horizontal actuator 60, until register marks 52 and 54 are in substantial alignment. A flow chart for the program of CPU 44 is shown in figure 8. First an image of the two overlying register marks is captured at step 62. Thereafter, the peak of the bright spot is determined at step 64. Many techniques, such as upsampling and interpolation, are known to those skilled in the art for determining the position of a peak from a set of data points. After the peak is determined, the error between the peak of the bright spot and the center of the register mark is calculated at step 66. If the error is at a minimum, an indication of registration is made at step 72. However, if the test at 68 indicates an error, the drive actuators are effected at step 70 to bring the two register marks 52, 54 into alignment. A new image is captured at step 62, and the process repeats. Register marks are also often printed alongside the completed color image in order to check the overall printing system accuracy. One pair of register marks may be used for each of cyan, magenta and yellow, the register mark for each color being printed over the register mark for black. A glance at the pairs of printed register marks will tell whether each of the color planes of the final image has been accurately aligned. In the case of a moving printed image, as in a high speed printing press, the printed register marks may be used to monitor the accuracy of the overall printing system. The image of the register marks may be captured by the use of a line sensor of photosensitive elements to scan the image of the register marks as the printed register marks move past the line sensor. The image processor detects misalignments by the position of the bright spot and initiates appropriate adjustments or alerts the operator to the status of the error condition. A register mark of the present invention does not require high intensity flash illumination systems or high resolution optics, in order to accurately determine alignment error from the register marks, thereby saving considerable cost and complexity in the design of automatic alignment and monitoring systems. Finally, register marks for use in accordance with the present invention may be used in projection alignment systems to align integrated circuit masks used in semiconductor manufacturing. In the latter case, the first register mark may be etched or deposited on the substrate, and the other register mark projected onto the first register mark. The finished semiconductor wafer may contain one or more pairs of overlying register marks so that an observer may determine at a glance whether each of the semiconductor masks of the final integrated circuit have been accurately aligned. A program written in QuickBASIC 4.5 to generate a register mark is given below: The z parameter is preferably 1.0 for the first (black plane) register mark 10, and 1.05 for the other register mark 12, corresponding to a 5 percent increase in frequency between the first and second register marks. The above program generates a register mark 200 x 200 pixels. The -.25 figure controls the size of the dots in the register mark pattern (i.e. the percent gray, not the frequency), and may be changed for different dot sizes, as desired.

Claims: A register mark arrangement comprising: a first register mark (10) including a first plurality of elements, the frequency of the spacial arrangement of which varies across the first register mark (10), and a second register mark (12) including a second plurality of elements, the frequency of the spacial arrangement of which varies across the second register mark (12), characterized in that: the elements of said first and second pluralities of elements comprise dots, the spacial frequency of each of said first and second pluralities of dots varies in the same sense with distance from the centre of said first and second register marks respectively, the spacial frequency of the dots of said second register mark is greater than the spacial frequency of the dots of said first register mark at corresponding areas of the first and second register marks, and said first and second pluralities of dots being arranged and said spacial frequencies chosen to produce an interference pattern including a bright spot at the centre of said register marks, when said register marks are disposed in overlaid relation, which is indicative of the alignment of said first and second register marks. A register mark arrangement according to Claim 1 in which said first and second pluralities of dots each comprises at least one line of dots in each of two orthogonal directions. A register mark arrangement according to Claim 1 in which each of said first and second pluralities of dots is a two-dimensional array of dots in which the spacial frequency of the dots decreases in opposite directions from the centre of the respective first and second register marks. A register mark arrangement according to Claim 1 in which each of said first and second pluralities of dots is a two-dimensional array of dots in which the spacial frequency of the dots increases in opposite directions from the centre of the respective first and second register marks. A register mark arrangement as claimed in Claim 1 in which each of said first and second pluralities of dots is a two-dimensional array of dots in which the spacial frequency of the dots is a function of the radial distance from the centre of the register mark. A register mark arrangement as claimed in Claim 5 in which in each of said first and second pluralities of dots the spacial frequency decreases with radial distance from the centre of the first and second register marks respectively. A register mark arrangement as claimed in Claim 5 in which in each of said first and second pluralities of dots the spacial frequency increases with radial distance from the centre of the first and second register marks respectively. A register mark arrangement as claimed in any preceding claim in which the spacial frequency of the dots of said second plurality is about 5% greater than the spacial frequency of the dots of said first plurality at corresponding areas of said first and second register marks. A register mark arrangement as claimed in any one of Claims 1 to 8 in which said first register mark is printed and said second register mark is projected thereon. A register mark arrangement as claimed in any preceding claim in which said first register mark is printed and said second register mark is printed thereover. A method of aligning first and second objects in which said first and second objects carry first and second register marks respectively for locating in overlaid relation to determine the alignment of said first and second objects, characterized in that: said first and second register marks provide a register mark arrangement which is in accordance with any one of claims 1 to 9, and wherein relative movement is effected between said first and second objects to bring the interference pattern between said first and second register marks to a condition exhibiting said bright spot at the centre of the register marks indicative of alignment thereof. A method as claimed in Claim 11 in which said first and second objects are layers intended to be brought into aligned superposed relation. A method as claimed in Claim 12 in which said first and second objects are associated with first and second colour planes in the printing of an image. A method as claimed in Claim 12 in which said first and second objects are respective colour plane negatives used in the process of burning printing plates for colour printing. A method as claimed in Claim 12 in which said first object is a semiconductor substrate and the second object is a mask for use therewith. A method as claimed in Claim 12 in which said first and second objects are respective masks for use in formation of an integrated circuit on a substrate. A method as claimed in any one of Claims 11 to 14 in which the displacement of the bright spot from the centre of the register marks is measured as an indication of the amount and direction of misalignment of the marks, and said relative movement is effected in accordance with said measurement. A method as claimed in Claim 17 in which the measurement of the displacement comprises the steps of: capturing an image of the interference pattern in a memory, determining the position of peak of the bright spot, calculating the error between the position of said peak and the centre of one of the register marks, ascertaining if the error is at a minimum and if so indicating alignment of the first and second register marks. A method of monitoring the alignment of first and second objects in which said first and second objects carry first and second register marks respectively for locating in overlaid relation to determine the alignment of said first and second objects, characterized in that: said first and second register marks provide a register mark arrangement which is in accordance with any one of Claims 1 to 10, and the method comprises the steps of detecting the position of the peak of said bright spot, calculating the error between the position of said peak, and the centre of one of the register marks, and ascertaining if the error is at a minimum to indicate alignment of said first and second register marks. A method as claimed in Claim 19 in which the detection of the position of the peak of said bright spot includes capturing in memory an image of the interference pattern between said first and second register marks. Apparatus for monitoring the alignment of first and second objects carrying first and second register marks respectively which register marks are located in overlaid relation, characterized in that: said first and second register marks provide a register mark arrangement which is in accordance with any one of Claims 1 to 10, and said apparatus comprises: detection means for detecting the position of the peak of said bright spot, means for calculating the error between the position of said peak and the centre of one of the register marks, and means for ascertaining if the error is at minimum to indicate alignment of said first and second register marks. Apparatus for aligning first and second objects carrying first and second register marks respectively, which register marks are located in overlaid relation, characterized in that: said first and second register marks provide a register mark arrangement which is in accordance with any one of Claims 1 to 9, and the apparatus comprises: detection means for detecting the position of the peak of said bright spot, means for calculating the error between the position of said peak and the centre of one of the register marks, means for relatively moving said first and second register marks in dependence upon said error in order to minimize said error indicative of alignment of said first and second register marks. Apparatus as claimed in Claim 21 or 22 in which said detection means includes means for capturing in memory an image of the interference pattern between said first and second register marks. Apparatus as claimed in Claim 21, 22, or 23 wherein said apparatus is for colour printing and said first and second objects are associated with first and second colour planes in the printing of an image. Apparatus as claimed in Claim 21, 22 or 23 wherein the apparatus is for the production of printing plates for colour printing and said first and second objects are respective colour plane negatives for use in the process of burning respective printing plates. Apparatus as claimed in Claim 21, 22 or 23 wherein the apparatus is for the production of integrated circuits and said first object is a semiconductor substrate and the second object is a mask for use therewith. Apparatus as claimed in Claim 21, 22 or 23 wherein the apparatus is for the production of integrated circuits and said first and second objects are respective masks for use in the formation of an integrated circuit on a substrate.

Inventors: LEVIEN RAPHAEL L, LEVIEN, RAPHAEL L.
Applicants: LEVIEN RAPHAEL L, LEVIEN, RAPHAEL L.
Application year: 1992
Main classification: G03F9
IPC codes: G03F7, B41F33, G03F9, H01L21