Abstract:
A method of patterning a wafer using four areas with differing exposure characteristics is disclosed. Two areas are phase shifted relative to the other two areas in order to create unexposed areas on the integrated circuit. Two different areas have polarizations orthogonal to each other, are frequency shifted relative to the two other areas, or are exposed by light at a time different than the two other areas to form exposed areas on the integrated circuit. The exposed areas are subsequently removed from the integrated circuit. In one embodiment, the four areas are on the same mask. The use of four areas with differing exposure characteristics allows for the patterning of more complicated and smaller geometric patterns on the integrated circuit than traditional patterning methods.

Description:
RELATED APPLICATION 
     This is related to U.S. patent application Ser. No. 09/727,666 filed Dec. 1, 2000, which is entitled “Method and Apparatus for Making an Integrated Circuit Using Polarization Properties of Light” and assigned to the assignee hereof. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to integrated circuit lithography and more particularly to the use of various properties of light in making the integrated circuits. 
     BACKGROUND OF THE INVENTION 
     A technique that is becoming important in the drawing of lines and features in an integrated circuit is a technique known as phase shift, which refers to a type of mask which provides for a 180 degree phase shift between various portions on the mask. That is, the light passing through the mask in some areas has a 180 degree phase shift in relation to light passing to the mask in other areas. This technique is very useful in drawing sharp lines by providing cancellation of the light at these phase shift boundaries where the light has the 180 degrees phase difference. One of the problems relating to this technology is that this 180 degrees phase difference also occurs in areas where lines are not necessarily intended to be drawn. The result has been that in these regions where there is no line intended to be drawn a second mask is utilized to remove or ensure exposure of the photoresist in those regions. In addition to the extra cost of this extra step, there are also alignment issues between the two mask steps. One of the difficulties with the use of phase shift masks has been the problems associated with the areas of interface between the different phase shifted areas. This difficulty has impeded the development of phase shift masks in the processing of integrated circuits. 
     One of the difficulties that arises is that the constraints on how the different features can be arranged and the typical routing techniques for maximizing the density of the packing of the different features has to be altered. Thus, this requirement of the second mask reduces the efficiency of the use of the available space that is available and creates additional concerns that must be accounted for in checking for violations of layout rules. Further, by virtue of having to utilize the second mask, there are alignment issues that create marginality problems that can result in the circuit not working as designed or the circuit designer having to take into account that these margin variations will occur in the operation. Furthermore, the two differing areas of phase shift limit the geometrical patterns that can be actually patterned in this manner. 
     Thus, there is not the full generality of layout possibilities that is present normally, without the use of phase shift masks. Thus there is seen a need for an improved ability to use phase shift masks in achieving circuit designs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not by limitation in the accompanying figures, in which like references indicate similar elements, and in which: 
     Shown in FIG. 1 is a layout which may be achieved using the present invention; 
     FIG. 2 is a table useful in understanding the invention; 
     FIG. 3 is another table useful in understanding the invention; 
     FIG. 4 is a cross section of a mask according to an embodiment of the invention, and 
     FIG. 5 is a cross section of a mask according to another embodiment of the invention. 
    
    
     Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention. 
     DETAILED DESCRIPTION 
     On a semiconductor wafer that is used for forming integrated circuits, features are typically formed by exposing photoresist according to a pattern. One such pattern is shown in FIG. 1 showing two T-gate transistors that are as close together as the particular technology allows. This demonstrates a need for four different states of light for providing the needed exposures. The overall result is achieved in one embodiment by using one light that has two states comprising a first polarization and two phases that are 180 degrees apart and another light that has two states comprising a different polarization and two phases that are 180 degrees apart. The result is that the desired lines are drawn using light of the same polarization but different phase and allowing an interface between light that has a different polarization without regard to phase. By virtue of having the different polarization, the light does not cancel at the interface even if the phase difference is 180 degrees. 
     Shown in FIG. 1 is an exemplary issue or problem and a solution according to the present invention. Shown in FIG. 1 is an integrated circuit  10  having a transistor  12  and a transistor  14 . Transistor  12  and transistor  14  are both T gate transistors. Transistor  12  has a first line  16 , second line  18 , and a third line  20 . Lines  16 - 20  comprise a gate of transistor  12 . Transistor  14  has a first line  22 , second line  24 , and a third line  26 . Lines  22 - 26  comprise a gate of transistor  14 . First line  16  and first line  20  can be viewed as one continuous line that is intersected by line  18  to form a T intersection. Similarly, line  22  and line  26  can be viewed as one continuous line intersected by second line  24  to form a T intersection. In this particular instance the distance between line  16  and line  22  is a minimum distance that is available to be drawn by the particular technology. In this particular instance these two T gate transistors  12  and  14  are as close together as can be achieved by the technology in question or the technology that is in use in this example. For example, this may be of the order of 0.1 micron. That may also be the width of lines  16 - 26 . 
     This particular structure has not effectively been achievable using conventional typical phase shift mask techniques, which provide just two types of light, one that is 180 degrees out of phase with the other. For example, in the region  30 , between transistors  12  and  14 , which is the minimum dimension region, would be one phase. For example, in order to draw the line  16 , the regions adjacent to line  16 , regions  30  and  32  plus  34 , would have to be 180 degrees out of phase. But, in order to draw line  18 , line  18  would also have to be bordered by two types of light that are 180 degrees out of phase. Similarly, for line  20 , the light in region  30  would have to be 180 degrees out-of-phase with the light bordering line  20 . With only two kinds of light that are 180 degrees out of phase, this required set of conditions cannot be achieved with a single exposure. 
     Shown in FIG. 1 are four states of light A, A′, B, and B′. Referring to FIG. 2 is a table which defines the different states of light that are available. In this example, state B has a polarization that is vertical and has a phase of zero degrees. State B′ has the same polarization but 180 degrees phase difference from that of state B. Another state is state A which has horizontal polarization with a zero degree phase shift, and state A′ has horizontal polarization but with 180 degrees phase shift in relation to state A. Thus, by having four states available, the two transistors  12  and  14 , in the close geometry shown in FIG. 1, are possible. These four states could be considered light with two attributes, wherein the light with each attribute is in two phases that are substantially 180 degrees out of phase with each other. In this polarization example, the two attributes are the two polarization orientations. 
     In this example, diagonal lines are drawn from the intersection of line  18  and lines  16  and  20  to form regions  32 ,  34 ,  36  and  38  with regard to transistor  12 . With regard to transistor  14 , diagonal lines are drawn at the intersection of line  24  and lines  22  and  26  to form regions  40 ,  42 ,  44  and  46 . Region  32  adjoins line  16 . Region  32  adjoins line  18 . Region  36  adjoins line  18 . Line  38  adjoins region  36  and line  20 . Similarly, region  40  adjoins line  22 , region  42  adjoins region  40  and line  24 , region  44  adjoins line  24  and region  46  adjoins region  44  and line  26 . Region  32  receives light in state B′ that is of the same polarization but different phase than that of state B, which is present in region  30 . The result is the cancellation of light at line  16  and thus the photoresist for line  16  is not exposed which is the desired result. Line  18  is formed by regions  34  and  36  receiving light in states A and A′ respectively. State A is of the same polarization as state A′. State A and A′, however, are 180 degrees out of phase with respect to each other. Thus, there is cancellation of light at line  18 , which is the desired result. Line  20  is formed by light received by regions  38  and  30  having the states of B′ and B, respectively. Thus, with states B and B′ having the same polarization but a 180 degree phase difference with respect to each other, cancellation occurs at line  20 . At the boundary of regions  32  and  34 , however, states of light B′ and A exist and are also 180 degrees different in phase but do not cancel. Because they are of different polarization, they do not destructively interfere, which is the desired result because that avoids forming an unwanted line. 
     Shown in FIG. 3 is a diagram which is similar to a logic diagram which shows the effect of the different states at boundaries of those states. For example, at the boundary between state A and A′ is a 1 which indicates cancellation. At the boundary between A and B there is a zero that indicates there is not cancellation and photoresist will be exposed. Similarly, a boundary of state A and B′ shows a zero that indicates light is not cancelled and photoresist is exposed. The effect of a boundary of states that show a 1 is that photoresist will not be exposed at that boundary and ultimately a line will be drawn. This may a polysilicon line, especially for minimum dimensions, but this is not limited to polysilicon and could be any type of drawn line. 
     Thus, now referring back to FIG. 1, line  22  shows a boundary of B and B′ that indicates cancellation and the formation of line  22 . Line  24  shows a boundary of states A and A′ and thus cancellation of light and the formation of line  24 . Line  26  has states B and B′ at its boundary and thus the cancellation of light and formation of line  26 . In this example, with only one type of light being available for region  30  due to the small dimension of region  30 , it is necessary to have all four states. An attempt to place different states in region  30  particularly at the intersection of line  24 , lines  22  and  26  would result in a design rule violation with the attendant reliability risks. 
     Shown in FIG. 4 is a mask  42  that provides states A, A′, B, and B′ in response to receiving polarized light. Mask  32  comprises a substrate  44 , an opaque region  46  on a top surface of substrate  44 , a birefringent region  48  on a bottom surface of substrate  44 , a birefringent region  50  on the bottom surface of substrate  44 , a birefringent region  52  on the bottom surface of substrate  44 , a phase shifting region  54  in the top surface of substrate  44 , and phase shifting region  56 . Phase shifting regions  54  and  56  are etched portions of substrate  44  that are etched to a depth that results in a 180 degree phase shift with respect to light passing through substrate  44  at its full thickness. The 180 degree phase differential is thus achieved by having a different thickness in mask  42  for the regions that are to be 180 degrees apart. Birefringent regions  48 - 52  contain a material, which is known as a birefringent material, that causes a 90 degree polarization rotation with respect to the received polarized light. State A is achieved by the polarized light passing through substrate  44  through its full thickness and not any birefringent material. State A′ is achieved by the polarized light passing through the substrate via a phase shifting region such as regions  56  but not through any birefringent material. State B is achieved by the polarized light passing through substrate  44  through its full thickness and through birefringent material. State B′ is achieved by the polarized light passing through the substrate via a phase shifting region such as regions  54  and  56  and through birefringent material. Light is blocked by opaque region  46 , which may be chrome. 
     Thus, for example, polarized light passing through substrate  44  and birefringent region  48  results in light in state B passing on to the integrated circuit that is being exposed. Similarly, polarized light passing through substrate  44  via a region  58  between birefringent layers  48  and  50  is in state A. Following from left to right then, state B is between region  58  and region  54 , state B′ is under region  54 , state B is between region  54  and region  46 , substantially no light is under region  46 , state B′ is under region  52 , and state A′ is adjacent to region  52  under region  56 . With opaque region  46  blocking the polarized light, there is not exposure to photoresist under opaque region  46  and the edges of opaque region  46  as passed on to the photoresist are sharp because the edges are exposed with the same polarization but different phases. The result is then a line in the integrated circuit caused by the pattern of opaque region  46 . 
     There will also be other lines caused by mask  42 . Both boundaries of region  54  will result in non exposure due to the cancellation of states B and B′. Thus, in the case of this mask  42 , there will be two lines formed that are two areas of an interface between states B and B′. Mask  42  demonstrates how the four states, A, A′, B, and B′ may be formed into any pattern. With states A, A′, B, and B′ available, any pattern is available to be made in the integrated circuit. 
     Shown in FIG. 5 is a mask  60  that has the same pattern of the four states as that of mask  42  of FIG.  4 . Mask  60  receives unpolarized light and converts it to two orthogonal directions of polarization. Mask  60  comprises a substrate  62 , a polarizing region  64  on a top surface of substrate  62 , a polarizing region  66  on the top surface of substrate  66 , a phase shifting region  68  in the top surface of substrate  62 , a phase shifting region  70  in bottom surface of substrate  62 , a polarizing region  72  on the bottom surface of substrate  62 , and a polarizing region  74  on the bottom surface of substrate  62 . Polarizing regions  64 ,  66 ,  72 , and  74  contain polarizing material that passes light that is polarized in a single direction. Polarizing regions  64  and  66  are oriented in a direction that is approximately 90 degrees different from that of regions  72  and  74 . The result is that light passing through polarizing regions  64  and  66  but not through polarizing regions  72  and  74  will be polarized. Similarly, light passing through regions  72  and  74  but not through regions  64  and  66  will also be polarized. Light passing through region  66  in that portion of region  66  that overlaps region  74  will be blocked. The overlap of the two polarizing regions of perpendicular orientation effectively forms an opaque region. 
     Thus, for example, the overlap of regions  66  and  74  achieves the same result as opaque region  46  in FIG.  4 . As a further example, the light passing through substrate  62  and region  72  results in state B and the light passing through substrate  62 , polarizing region  64 , and a region  76  between polarizing regions  72  and  74  results in light in state A. Light in state B is between phase shifting region  68  and polarizing region  64 . Light in state B′ is under phase shifting region  68 . Light in state B is between phase shifting region  68  and polarizing region  66 . Light in state A′ is under phase shifting region A′. Light in state A is adjacent to phase shifting region  70  and under polarizing region  66 . Thus, any pattern of the four states A, A′, B′, and B′ may be achieved. 
     Thus, masks  42  and  46  show two different ways of achieving four states in which there are two combinations in which, when forming a boundary, light is cancelled along the boundary and in which all of the other combinations are not destructive at their boundaries. These two examples of masks, masks  42  and  60 , each use polarized light to avoid destructive interference at phase boundaries that are not intended to have destructive interference while still achieving destructive interference at those boundaries where destructive interference is desired. There may be other ways to achieve this result. For example, two different colors (frequencies) of light may be used instead of different polarizations. This may be achieved by placing light filters on the mask. In such case, differing adjacent colors, with their differing frequencies, would not result in cancellation. Another alternative is to do exposures at different times. At a boundary where a line is not to be drawn, one side of boundary is exposed at one time and the other side at a subsequent time. Thus, even if the light on opposite sides of the boundary was of the same frequency and 180 degrees out of phase, there would not be destructive interference because these two states of the light would arrive at the photoresist at different times. Thus the first exposure would be to draw some precise lines and expose one side of the boundary of others where destructive interference is to be avoided. The second exposure would be to draw other precise lines and provide the complementary exposure to the boundaries that were half exposed during the first exposure. Thus, the equivalent of states A and A′ would be provided during the first exposure and states B and B′ would be provided during the second exposure. 
     With regard to the different time approach, there is the disadvantage of aligning consecutive masks, typically using a stepper. There is, however, also the advantage of being able to actually achieve some of the structures that are essentially impossible with the current approach of using the first mask step for all of the lines requiring phase shifting and a second mask for both removing the unwanted lines that are artifacts from the phase shifting done during the first exposure and patterning the routing polysilicon. That approach, for example, has not been demonstrated to be able to make T-gate transistors. Typical use of phase shifting masks is that polysilicon level where the gates are formed, but there is nothing in the embodiments described herein that restrict the use to polysilicon or the gate level. 
     The alignment issue that arises from using two masks can be alleviated by actually making two masks on the same reticle. The technique involves using one mask pattern for the first exposure and a second mask pattern for the second exposure but with no unloading and loading between the two mask exposures because both patterns are on the reticle that is loaded on the stepper. This is possible because it is the same photoresist layer that is being exposed by both exposures. Thus, many of the alignment issues that typically occur between mask steps are not present if the reticle contains both mask patterns. For example, the realignment of the reticle to the stepper is not required that is a major cause of alignment problems. Also the reloading process includes unloading and reloading the wafer. Thus the wafer has to be realigned, which is another major source of exposure to exposure alignment problems. These two major problems are thus avoided by putting both patterns, which are consecutively exposed, on the same reticle, The desired result is achieved by exposing the integrated circuit to one of the two patterns, moving the integrated circuit under the second of the two patterns, and then performing the subsequent exposure. 
     These three different approaches, polarizing, different colors, and different time, can also be implemented with reflective masks. For example, the incident light may be polarized and the surface could have a transparent polarizing material that is appropriately patterned. Similarly, the surface can be patterned to reflect different colors. Also in this reflecting case, the method of using different points in time for the exposures can be employed. 
     In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, the embodiments describe the utility of the invention in the context of forming gates, but the invention could be used for forming any feature such as, but not limited to, metal interconnect. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.