This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2005-287508 filed on Sep. 30, 2005, the content of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor device having high-density contacts each having an oval profile, and more particularly to the arrangement of the contacts.
2. Description of the Related Art
In semiconductor devices such as DRAM, the electric charge that accumulates on the capacitors of memory cells is used as an information source. The capacitor comprises a lower electrode, a dielectric mounted on the lower electrode, and an upper electrode mounted on the dielectric; these are arranged in a contact hole. The lower electrode connects with under-layer contact plug that is arranged at a bottom of the contact hole. The under-layer contact plug connects with an active region in the surface of a semiconductor board.
With the trend toward higher integration of semiconductor memory, the contact hole for forming the capacitor is being arranged in higher density. Further, with the trend toward further higher integration and greater miniaturization, demands are growing for high-density contact holes in smaller size and in higher density. A variety of designs have been developed regarding the arrangement and shape of these contact holes in order to respond to these demands.
In DRAM cells having a structure of COB (Capacitor Over Bit line) which is currently in vogue, capacitors are arranged above a bit line via capacitance contact plugs. In this case, the plane shape of the contact hole, in which the capacitor is arranged, is an oval.
However, when a hole having an oval plane shape for forming a contact hole is formed on a thick insulating layer by dry etching, a large degree of bowing occurs on the minor axis side even when the difference between the major axis and minor axis is slight. Bowing refers to a phenomenon in which the excessive progression of etching in a portion in the side-wall direction of the hole causes a concave shape in the side walls. This bowing is attended by a phenomenon in which the diameter of the hole bottom is reduced in the direction of the minor axis.
It turned out that bowing tends to occur in the direction of the minor axis when etching holes in silicon oxide, particularly etching holes in a pattern having a high aspect ratio, as in forming this type of contact. In contrast, bowing tends not to occur in the major axis direction. This distortion is believed to occur because the orbits of the ions for etching are distorted by the charge of the mask for hole formation, whereby etching is accelerated. In other words, it is believed that the orbits of ions tend to be distorted in the direction of the minor axis in which the mask is narrow, whereby bowing tends to occur in the direction of the minor axis. It is further believed that, due to the distortion of the orbits of ions, sufficient energy fails to reach the bottoms of holes and thus reduces the diameter in the direction of the minor axis.
Bowing also occurs in contacts having a circular plane shape, and methods have been proposed for preventing this occurrence. In JP-A-H08-191062, dry etching is carried out in two stages. Isotropic dry etching is first carried out as far as midway through the direction of film thickness of the dielectric, and anisotropic dry etching is then carried out to the end of the dielectric. In JP-A-2005-229052, anisotropic dry etching is first carried out as far as midway through the interlayer dielectric under the condition of low etching selectivity of the photoresist and interlayer dielectric. Anisotropic dry etching is next carried out as far as the electrical connection region or the electrical wiring of the active elements under the condition of high etching selectivity of the photoresist and interlayer dielectric. Alternatively, in JP-A-2004-335526, the first etching is halted at a depth at which bowing does not occur to form holes. An etching protection film is next formed in portions of the side walls of these holes at which bowing is expected to occur when progressing further in the formation of holes. The second etching is then carried out to form the holes to the prescribed depth. Fine holes having an aspect ratio of 13 or more are formed by this method.
The methods for restricting bowing described in the previously mentioned patent documents take as object contacts having a circular outer shape, and the steps of these methods are complicated, thereby reducing accuracy of control in forming a hole. Due to these factors, these methods are not used for high-density contacts having an oval profile in semiconductor devices, and in particular, high-density contacts for DRAM capacitors.
As shown in
In the present specification, the separation width means the thickness of the walls composed of the remaining portion of substrate between adjacent contact holes 50, and consequently, between contacts that are formed inside contact holes 50. The minimum separation width refers to the thickness of the portions of these walls that-are thinnest. In this context, the separation width and the minimum separation width do not include the influence of bowing.
As shown in
The following explanation regards hole layout 5 for high-density contacts of a semiconductor device of the related art and the state of occurrence of bowing by way of a specific fabrication example. In this fabrication example, minor axis 52 of the oval profile of contact holes 50 is 204 nm, major axis 51 is 240 nm, and minimum separation width 56 is 67 nm (dimensions in the mask pattern). The vertical structure of the substrate and mask by which contact holes 50 are formed is: Poly-Si mask (800 nm)/P-TEOS (3000 nm)/P-SiN (50 nm). When etching SiO2, a two-frequency parallel plate RIE (Reactive Ion Etching) device is used, and processing is carried out in an atmosphere of C4F6/Ar/O2 gas at 25 mTorr. For SiN etching, the same device was used and processing was carried out in an atmosphere of CHF3/Ar/O2 gas at 25 mTorr. In the interest of appraising shorts between adjacent contact holes 50, 20% overetching was carried out in the etching of SiO2.
Contact holes 50 were processed to a depth of 3000 nm under these conditions. Here, bowing occurred at positions at a depth of approximately 250 nm as shown schematically in
As can be seen from the above-described fabrication example, generally, when contact holes 50 for oval contacts are formed by dry etching, as a result of the nature of dry etching, conspicuous bowing is apparent in the direction of minor axis 52 even though almost no bowing occurs in the direction of major axis 51. Thus, in an arrangement in which the distance between side surfaces in the direction of minor axis 52 of contact holes 50 is the minimum separation width 56, the wall thickness between adjacent contact holes 50 is extremely small at the positions of bowing portions 55. In other words, in such an arrangement, positions where walls that separate adjacent contact holes 50 are thinnest are subject to a relatively high degree of reduction due to bowing, and there is consequently a high danger of shorts between contact holes 50 at these reduced portions.
It is an object of the present invention to provide a semiconductor device having high-density contacts that have an oval profile wherein the semiconductor device, without necessitating a special etching step, enables a suppression of the occurrence of shorts between adjacent contact holes despite the occurrence of bowing.
This semiconductor device of the present invention includes a substrate on which a plurality of contact holes, a plane shape of each of which is a oval, are formed, and in each contact hole a contact is formed having an oval profile that corresponds to the contact hole. The position on the perimeter of at least one of oval at which the separation width between that oval and an adjacent oval is a minimum is separated by at least a prescribed spacing from the intersection between the perimeter of that oval and the minor axis of that oval.
The prescribed spacing is preferably equal to a distance corresponds to a sum of a maximum growth width of bowing at an intersection between the perimeter of said oval and a minor axis of that oval, and a maximum growth width of bowing at position on the perimeters of adjacent said oval at which the separation width between adjacent said ovals is a minimum.
A position on a perimeter of each of said ovals at which a separation width between said oval and an adjacent said oval is a minimum is preferably separated from an intersection between the perimeter of that oval and a minor axis of that oval
The extended line of the minor axis of each oval preferably does not intersect with the perimeter of the oval that is closest to that oval.
The above-described arrangement of ovals can be realized by adjusting the direction of the major axes or minor axes of the ovals. In particular, the extended lines of the minor axes of a pair of ovals arranged above the same active region preferably intersect at an angle of at least 45°.
The contact holes may constitute holes in each of which a capacitor for DRAM is arranged.
Thus, despite the occurrence of bowing at the time of etching in holes for forming oval-shaped contacts, the present invention can suppress the reduction of wall thickness between contacts caused by this bowing. The present invention can therefore suppress the occurrence of shorts between adjacent contacts, and further, shorten the distance between contacts while suppressing the occurrence of shorts to thus raise the density of the arrangement of contacts.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.
In the present invention, in response to the above-described problems, layout is realized in accordance with the following two principles: (1) the separation width between contact holes is not to become a minimum between side surfaces in the direction of the minor axes; (2) contact holes are to be arranged such that the separation width between contact holes becomes a minimum between side surfaces as close as possible to the direction of the major axes. In this way, the present invention can realize processing that limits the reduction of the margin for suppressing shorts between contact holes, this reduction being caused by bowing at the time of etching. In particular, the present invention, to the greatest possible extent, makes those positions at which the separation width between adjacent contact holes becomes a minimum the positions in the direction of the major axis in which virtually no bowing occurs and thus enables a marked increase in the margin for suppressing shorts.
Hole layout 1 for high-density contacts in the present embodiment shown in
When the dimensions of each contact hole 10 and the distance between the center points of each contact hole 10 are identical to those in the related art, minimum separation width 16 in the present embodiment is 58 nm, which is slightly smaller than in the related art.
Bowing occurs at positions at a depth of approximately 250 nm after processing contact holes 10 to a depth of 3000 nm, as in the related art; and
The present embodiment thus enables an increase in the minimum wall thickness between contact holes 10 when the distance between the center points of contact holes 10, i.e., the degree of density of contacts 13 or arrangement density, is made the same as in the related art. As a result, the present embodiment can suppress the occurrence of shorts, and further, raise the arrangement density of contacts 13 while taking into consideration the fabrication error to ensure sufficient wall thickness to enable suppression of the occurrence of shorts. In this time, in order to achieve the maximum possible increase in arrangement density while ensuring sufficient wall thickness to enable suppression of the occurrence of shorts, a spacing from a position on a perimeter of contact hole 10, at which a separation width between that contact hole 10 and an adjacent contact hole 10 is a minimum, to an intersection between the perimeter of that contact hole 10 and a minor axis of that contact hole 10, is preferably set to a distance corresponds to a sum of a maximum growth width of bowing at an intersection between the perimeter of that contact hole 10 and a minor axis of that contact hole 10, and a maximum growth width of bowing at position on the perimeters of adjacent contact hole 10 at which the separation width between adjacent contact holes 10 is a minimum.
In the present embodiment, a configuration was shown in which, for all contact holes 10, the intersections between the perimeters and minor axes 12 are positioned sufficiently far from positions on the perimeters corresponding to minimum separation width 16. However, when a relatively large spacing can be ensured between a number of contacts 13, the intersections between the perimeters and minor axes 12 for these contacts 13 may coincide with the positions on the perimeters that correspond to minimum separation width 16. In this case, minimum separation width 16 between these contacts 13 is preferably set to a distance obtained by multiplying a prescribed safety factor by twice the maximum growth width of bowing that is expected to occur in the direction of the minor axes of contact holes 10.
Hole layout 2 for high-density contacts in the present embodiment shown in
As in the related art, after contact holes 20 have been processed to a depth of 3000 nm, bowing occurs at positions at a depth of approximately 250 nm, and
Although explanation has been given for two embodiments, the first and second embodiments, the present invention is not limited to these forms, and as previously explained, the present invention is based on the two principles: (1) contact holes are to be arranged such that the separation width between contact holes is not a minimum between side surfaces in the direction of minor axes; (2) contact holes are to be arranged such that the separation width between contact holes is a minimum between side surfaces as close as possible to the direction of major axes. By means of these principles, processing can be realized that decreases the reduction of the margin for suppressing shorts between contact holes, this reduction being caused by the bowing that occurs in etching. In particular, as in the second embodiment, the margin for suppressing shorts can be greatly increased by adopting an arrangement such that, to the greatest extent possible, the separation width between contact holes is a minimum between side surfaces close to the direction of the major axes, at which virtually no bowing occurs.
The adoption of this type of layout enables processing of high-density contacts having a high aspect ratio, and in particular, high-density contacts for DRAM capacitors, which demand fine and precise processing, while suppressing malfunction that results from bowing.
While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.
Number | Date | Country | Kind |
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2005-287508 | Sep 2005 | JP | national |