Method of fabricating reduced critical dimension for conductive line and space

[0002] This application claims the priority benefit of Taiwan application serial no. 88110532, filed Jun. 23, 1999.

[0003] 1. Field of the Invention

[0004] The present invention relates to a fabrication method for a semiconductor device. More particularly, the present invention relates to a fabrication method for reducing the critical dimensions for the conductive lines and the space in between.

[0005] 2. Description of the Related Art

[0006] In the fabrication of a semiconductor device, the resolution of the photolithography technique is limited, even though the short wave length deep ultraviolet (DUV) light is used. For example, the photolithography resolution of a DUV with a wavelength of 248 nm, is around 0.18 to 0.2 mm. A further increase in the density of a semiconductor device is thereby limited by the resolution of the current photolithography technique.

[0007] The present invention provides a method of fabricating a reduced critical dimension for the conductive line and the space to overcome the resolution limitation of the photolithography technique. This invention includes forming sequentially a material layer and a mask layer on a substrate, wherein the material layer, for example, is a conductive layer. A first taper etching is conducted to form multiple first openings with the cross-sections of the openings being tapered off from top to bottom exposing the surface of the material layer. A sacrificial layer is then formed on the exposed surface of the material layer. A second taper etching is conducted on the exposed mask layer to form multiple second openings with the cross-sections of the openings being tapered off from top to bottom. The sacrificial layer is then removed. Thereafter, an anisotropic etching is conducted to remove the exposed material layer, with the mask layer serving as a hard mask, forming multiple conductive lines followed by a removal of the mask layer.

[0008] According to the present invention, the critical dimension of the conductive line and of the space can be reduced. The conductive line width is a sum of the distance of the sidewalls of the first and the second openings being tapered off after the first and the second taper etching. The space between the conductive lines is at least a line width less than the regular photolithography resolution.

[0009] It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.

[0010] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,

[0011] FIGS. 1A to 1G are schematic, cross-sectional views of a semiconductor device showing the fabrication method for reducing the critical dimension of the conductive line and the space.

[0012] FIG. 2 is a schematic, cross-sectional view showing the step of the second taper etching, in which the sacrificial layer formed is higher than the mask layer.

[0013] FIGS. 1A to 1G are schematic, cross-sectional views of a semiconductor device showing the fabrication method for reducing the critical dimension of the conductive line and the space.

[0014] Referring to FIG. 1A, a conductive layer 110 and a mask layer 120 are sequentially deposited on a substrate 100. The conductive layer 110, including doped polysilicon, aluminum and copper type of conductive materials, are formed by sputtering or chemical vapor deposition. The mask layer 120, on the other hand, including silicon oxide and silicon nitride, is made of different materials and comprises a different etching selectivity from the conductive layer 110.

[0015] A photoresist layer is formed on the mask layer by spin-coating, and is then patterned by conventional photolithography and etching process into a patterned photoresist 130 on the mask layer 120. The pattern of the patterned photoresist 130 is defined by a width “a” of the photoresist and a width “b” of the interval spacing separating two adjacent portions with a width “a” of the patterned photoresist.

[0016] With the photoresist layer 130 serving as a mask, as shown in FIG. 1B, a first taper etching is conducted on the mask layer 120 to form a patterned mask layer 120awith an opening 140 in which its cross-section is tapered off from top to bottom. If a sidewall of the opening 140 is reclined as in FIG. 1B, the width of the opening 140 at the bottom is reduced by “c”. If both sidewalls of the opening are reclined and the width of the opening is also reduced by “c” at each sidewall, the width of the opening is totally reduced by “2c”. Since the width of the opening 140 at the top is “b”, the width of the opening at the bottom becomes “b−2c”. Correspondingly, the width of the bottom of the patterned mask layer is increased to “a+2c”. After which, the photoresist layer 130 is removed.

[0017] If the mask layer 120 is silicon oxide, a reactive ion etching, for example, can be conducted using the CH₂F₂/C₄F₈/Ar ion as a gas source at flow rates of 4 to 8 sccm, 8 to 14 sccm and 400 to 500 sccm respectively and at a reaction chamber pressure of approximately 30 to 50 mtorr. The flow rate of CH₂F₂is controlled such that the degree of inclination of the opening 140 sidewall is adjusted to approximately between 85 to 87°. Hence, the degree of inclination and the thickness of the mask layer 120 determine the extent of “c”, wherein c is degree of reduction of the width of the opening 140 at the bottom due to a relining sidewall.

[0018] As shown in FIG. 1C, a fluid material, with a good void filling capability, is formed on the conductive layer 110, forming a planarized sacrificial layer 150. The sacrificial layer 150, for example, an organic material used for the bottom anti-reflection coating (BARC), is of a different material and comprises a different etching selectivity from the mask layer 120a.In the current preferred embodiment, the sacrificial layer is formed covering the conductive layer 110 and the mask layer 120a.The portion of the sacrificial layer 150 above the mask layer 120ais then removed, for example by an etch back process, leaving only the portion of the sacrificial layer 150 having the same height as the mask layer 120a.This invention, however, is also applicable to have the sacrificial layer 150 formed with its height higher or lower than the mask layer 120a.

[0019] If the sacrificial layer 150 is an organic bottom anti-reflection coating, it is removed by, for example, dry etching using an end-point detector (EPD) system with the mask layer 120aserving as an etch stop. The gas source for dry etching plasma includes CO/C₄F₈/Ar/O₂at flow rates of 20 sccm to 50 sccm, 10 sccm to 14 sccm, 100 sccm to 200 sccm, and 10 sccm to 20 sccm respectively, and at a reaction chamber pressure of approximately 40 to 50 mTorr.

[0020] As illustrated in FIG. 1D, a second tapered etching is further conducted to etch the mask layer 120a(as shown in FIG. 1C), using the sacrificial layer 150 as a hard mask layer, to form the openings 160 and the patterned mask layer 120b.If the mask layer 120bis silicon oxide and the sacrificial layer 150ais an organic BARC, the etching conditions can be similar to those illustrated in FIG. 1B, wherein the etching selectivity of silicon oxide to BARC can reach a rate 6:9. The bottom side of each mask layer 120bthus has a width “2c”. Correspondingly, the top of the opening 160 has a width a while the bottom has a width (a−2c), due to tapered etching.

[0021] If the sacrificial layer 150 formed is lower than the mask layer 120 (as in FIG. 1B), the width of the bottom of each mask layers 120bis thus less than “2c” as illustrated by the dotted line in FIG. 1D.

[0022] As shown in FIG. 2, when the sacrificial layer 150 (as in FIG. 1B) is higher than the mask layer 120a(as in FIG. 1B), the sacrificial layer 150 is patterned and a portion of the sacrificial layer 150 is removed to expose a portion of the mask layer 120asurface, using the mask layer 120aas an etch stop. The second taper etching is then conducted to etch the mask layer 120a(as in FIG. 1C) to form the mask layer 120b,wherein the width of the bottom of each mask layer 120bis greater “2c”. As illustrated in FIG. 2, the portion of the mask layer (as indicated by the shade area in FIG. 2) is removed by taper etching to form a mask layer 120bwith the width of its bottom equal to “d”, where “d” is greater than “2c”.

[0023] Again referring to FIG. 1D where the second etching step is being performed, if the etching condition, such as the flow rate of CH₂F₂is altered, the degree of inclination of the sidewalls of the opening 160 will be different from that of the first etching step. If the bottom of the opening 160 is tapered off to a value of “a−2e”, the width of the bottom of each mask layer 120bis “c+e”.

[0024] Continuing to FIG. 1E, the remaining sacrificial layer 150ais removed, and the distances between the adjacent mask layers is either “a−2c” or “b−2c”, if the second taper etching conditions are same as those of the first taper etching step. If the sacrificial layer 150ais an organic BARC, a common dry etching for removing a photoresist can be applied using oxygen plasma.

[0025] Referring to FIG. 1F, the exposed conductive layer 110 is removed, with the mask layer 120bserving as hard masks, to form a conductive line 110awith a line width of “2c”. If the conductive layer 110 is, for example, doped polysilicon, a reactive ion etching using chlorine ion as a gas source can be conducted, in which the etching selectivity of doped polysilicon to silicon oxide is about 40 to 100 with the mask layers 120bbeing silicon oxide. If the conductive layer 110 is aluminum and the mask layer 120bis silicon oxide, a reactive ion etching using chlorine ion as a gas source still can be conducted with an etching selectivity of aluminum to silicon oxide being approximately 10 to 16.

[0026] As shown in FIG. 1G, the mask layers 120bare removed, leaving the conductive lines 110aon the substrate 100. If the mask layer 120bis silicon oxide, it is removed, for example, by a reactive ion etching using a fluorine ion gas source or a hydrogen fluoride (HF) solution.

[0027] Based on the present invention, when the conductive layer 110 is doped polysilicon and the mask layer 120 is silicon oxide, the line width of the doped polysilicon conductive line, determined under a scanning electron microscope (SEM), is approximately 0.079 micron. The distance between the adjacent doped polysilicon conductive lines is approximately 0.204 micron.

[0028] According to the preferred embodiment of the present invention, the present invention overcomes the limitations of the photolithography resolution in fabricating a conductive line with a reduced critical dimension. Hence, the device density and the efficiency of the semiconductor circuitry are greatly increased. Although in the above embodiment, the present invention has been described with respect a conductive line with a reduced critical dimension, the invention is applicable also to other devices of a semiconductor device with reduced critical dimensions.

[0029] It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.