Nested overlay measurement target

A mask overlay measurement target includes nested boxes on three levels or has adjacent boxes sharing a common side, saving substantial area. The nested overlay measurement target also provides savings in measurement time since multiple overlay combinations can be measured at once. The nested target provides more level-to-level overlay information than has been available with standard box-in-box targets. The nested boxes are also used on a single level to provide area savings for stepper field placement metrology.

FIELD OF THE INVENTION

This invention generally relates to overlay measurement targets. More particularly, it relates to overlay measurement targets for semiconductor wafers. Even more particularly, it relates to an improved target that saves room in kerf areas of a wafer.

BACKGROUND OF THE INVENTION

Semiconductor wafer fabrication involves a series of photolithographic steps to provide patterned layers of material one over another. The layers must be carefully, and precisely aligned to each other so that the final structure functions as planned. Structures are included in each layer for measuring how much that layer is out of alignment with a previously formed layer. Structures may also be provided for measuring how much a later formed layer is out of alignment with that layer. These structures are typically boxes of different size. The x and y misalignment is measured by how much a box on one level is not centered within a larger box on another level in both x and y directions.

A problem with this technique became evident with the advent of damascene wiring that is fabricating with chemical mechanical polishing. Lift off and back etch processes permitted a large area blanket metal layer to cover targets on lower levels. Thus, an overlay target could be stacked without interference from underlying targets. With damascene wiring, no large area blanket metal layer could be used so underlying targets would interfere with a stacked target. A larger amount of area in the kerf region between chips has been needed for overlay measurement targets to overcome this problem, crowding out other test structures. In addition, as the number of masks and wiring levels increases the number of overlay alignment measurement targets has increased, using up more and more area in the kerf for both standard etch and damascene processed layers. Thus, a better solution for overlay measurement targets is needed that avoids using a large amount of kerf space, and this solution is provided by the following invention.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a way to reduce the area for overlay measurement targets.

It is a further object of the present invention to provide a mark on each layer of three layers that combine to provide a three level nested target for providing x or y coordinate information for the three layers relative to each other.

It is a further object of the present invention to provide marks on each layer of at least three layers in which a mark on one level includes at least two nested marks on other levels that are not nested with each other.

It is a further object of the present invention to provide nested marks for measuring stepper field placement on adjacent overlapping fields.

It is a further object of the present invention to provide a mark on one level having a plurality of boxes and marks within those boxes on two or more other layers.

It is a further object of the present invention to provide marks on each layer of at least three layers in which at least two marks are nested and two marks on one level share a common edge. The nested target provides x or y coordinate information for the three layers relative to each other.

It is a feature of this aspect of the present invention that the targets provide alignment coordinate information for at least three levels.

It is an advantage of the present invention that the nested overlay measurement targets use less space than would be required by separate overlay measurement targets for each pair of levels.

These and other objects, features, and advantages of the invention are accomplished by a multilayer structure, comprising marks on each layer of three layers, said marks combined to provide a nested overlay measurement target, said nested measurement target comprising a first mark on a first layer in a second mark on a second layer in a third mark on a third layer, said nested overlay measurement target for providing x or y coordinate information for the three layers relative to each other.

Another aspect of the invention is accomplished by a multilayer structure, comprising marks on each layer of at least three layers, said marks combined to provide a first and a second nested overlay measurement target, said first and said second nested measurement targets sharing a common edge, said first nested measurement target comprising a first mark on a first layer in a second mark on a second layer, said second nested measurement target comprising a third mark on a third layer in a fourth mark on a fourth layer, said nested overlay measurement target for providing x or y coordinate information for the three layers relative to each other.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors discovered that overlay measurement target 18 can include more than two levels, as shown in FIG. 1 . By providing nested targets formed of boxes 20 , 22 , and 24 substantial space can be saved. Box 20 on a first level is shown inside box 22 on a second level which in turn is inside box 24 on a third level. The mask levels can include silicon and/or metalization levels. For example the first level can be the first level of metal, M 1 , the second level can be the first stud level, V 1 , and the third level can be the second level of metal, M 2 .

A standard overlay target design uses a 13 13 um inner box and a 26 26 um outer box. The present inventors found that they could provide two boxes nested inside a 28 28 um box, effectively providing the information that required two standard 26 26 um boxes in one slightly larger target. Thus, the new target nearly halves the space for alignment metrology.

The space savings possible for a complete set of mask levels is shown in FIGS. 2 a and 2 b . The 36 standard two-level measurement targets 18 a that were needed to provide for the needed measurements, shown in FIG. 2 a , could be replaced by 14 three level measurement targets 18 b shown in FIG. 2 b , again saving about half the space.

Boxes 20 , 22 , 24 of target 18 need not be complete; bars 26 can replace full sides, as shown in FIG. 3 a or FIG. 3 b . A box can be larger or smaller than a box on a lower level.

An alternate scheme to the fully nested targets for measuring misalignment of two levels to a third reference level is shown in FIG. 4. A scheme to measure misalignment among five levels is shown in FIG. 5 in which marks on four of the levels can all be measured to a mark on a fifth level. A connected box mark is used on the reference level. This scheme is particularly useful for conserving space while measuring a number of levels referenced to a single level. In FIG. 5 , the alignment of marks 30 , 32 , 34 , and 36 on four different levels can all be measured with reference to mark 28 on a fifth level. Mark 28 can be considered to have side 28 a that is shared by two adjacent overlay measurement targets. In addition to referencing mark 28 , the alignment of level 30 to level 32 and of level 34 to level 36 can also be measured in this compact structure. Such a structure may be used for measuring misalignment to a single level, such as the very first mask used in the process in the silicon level. For example, level 28 may be the Rx mask, which is the first masking level at IBM. Levels 30 and 32 may be the next two levels above Rx, which may be designated AB and NW. Levels 34 and 36 may be the next two levels above NW, which may be designated CA and PC.

As in FIGS.3 a and 3 b , boxes 28 , 30 , 32 , 34 , and 36 need not be complete; bars 26 can be used as shown in FIGS. 6 a or 6 b . Boxes can also have wide sides 40 , and center box 42 can cover the entire area within, as shown in FIG. 7 .

The common side idea of FIGS. 4 , 5 , 6 a , 6 b and 7 can be extended to provide a large number of levels 48 , 50 , 52 , 54 , 56 all referenced to a single mask level 58 , as shown in FIG. 8 a . Substantial savings in area are obtained with this arrangement as compared with the prior art that provides alignment measurement for as many levels shown in FIG. 8 b . The same information is provided by FIGS. 8 a and 8 b but the layout of FIG. 8 a is much more compact because of nesting and sharing one to four common sides reduces the number of separate targets needed.

Nesting also permits area savings for measuring first level field placement as shown in FIG. 9 a . The measurement is to ensure that adjacent stepper fields were placed as desired to avoid magnification and rotation errors in stepping from one field to the next field. Substantial savings in area are obtained with this arrangement as compared with the prior art that provides first level field placement information shown in FIG. 9 b . The same information is provided by FIGS. 9 a and 9 b but the layout of FIG. 9a is much more compact because of nesting. Conventionally, targets 60 , 62 , 64 , and 66 are located in fields A, B, C, and D, as shown in FIG. 9 b . Target 60 includes box A in box B; target 62 includes box B in box D; target 64 includes box C in box A; and target 66 includes box D in box C. Thus, four separate targets are needed to provide information for the four fields relative to each other. In the nested target of the present invention, target 68 includes box A in box B in box C in box D , as shown in FIG. 9 a . Any variation in magnification or rotation that could be observed in the four targets of FIG. 9 b can be seen in the single target of FIG. 9 a . In addition there are four additional pieces of information corresponding to the placement of diagonal fields. The invention takes advantage of the fact that adjacent stepper fields overlap so the four marks can be provided in one target.

The present invention provides advantage in addition to saving space; it provides more level to level data and it also can save time since the various overlay combinations can all be measured in a single measurement. For example, in FIG. 1 , M 2 to M 1 , M 2 to V 1 , and M 1 to V 1 can all be measured in one measurement scan. Three targets would have been needed in the standard box-in-box scheme to provide the same amount of data, and the measurement would have taken three times the time. The invention is applicable to standard metallization, damascene, and dual damascene technology for silicon, implant, gate, isolation, metalization, and insulation levels.

While several embodiments of the invention, together with modifications thereof, have been described in detail herein and illustrated in the accompanying drawings, it will be evident that various further modifications in the design of the combined mark are possible without departing from the scope of the invention. For example, more than three boxes can be nested in FIGS. 1 and 5 . Nothing in the above specification is intended to limit the invention more narrowly than the appended claims. The examples given are intended only to be illustrative rather than exclusive.