A method of forming an interconnection between conductive levels

Abstract: A method of forming an interconnection between conductive levels is described in which a first conductive level (4) is provided on a surface of a substrate body such as a semiconductor body (1) so that the first conductive level (4) has a contact area (10). Passivating material (6) is provided on the surface of the body (1) to cover the first conductive level (4) and the contact area (10) then exposed by opening in the passivating material (6) a window (9) larger than the contact area (10) so that there is a gap (11) between the periphery (9a) of the window (9) and a side wall (5a) of the first conductive level (4) bounding the contact area (10). After opening the window (9) material (13b) is provided in the gap (11). Preferably, the material is provided by applying and then solidifying a spin-on-glass and subsequently etching back the spin-on-glass to expose the contact area. A second conductive level (7) is then provided on the smoothed surface to smooth the surface between the periphery (9a) of the window (9) and the contact area (10) so that part of the second conductive level (7) contacts the area (10) within the window (9).

Description: This invention relates to a method of forming an interconnection between conductive levels, which method comprises depositing and then etching conductive material to define a first conductive level on a surface of a substrate body so that the first conductive level has a contact area, providing insulating material on the surface of the body to cover the first conductive level, exposing the contact area by anisotropically etching the insulating material to form a window in the insulating material larger than the contact area so that there is a gap between the periphery of the window and a side wall of the first conductive level bounding the contact area, the anisotropic etching leaving portions of the insulating material on the said side wall thereby reducing the gap, and depositing and then etching conductive material to define a second conductive level on top of the insulating material so that part of the second conductive level contacts the contact area within the window. Such a method is disclosed in US-A-4594606. As described in US-A-4594606, the window in the insulating material covering the first conductive level is opened by etching the insulating material, in the arrangement described silicon dioxide, anisotropically, using reactive ion etching, so as to expose the upper surface of the first conductive level and leave portions or fillets of silicon dioxide on side walls of the first conductive level bounding the contact area. US-A-4594606 stresses the importance of the fillets provided on the edges of the contact area being smooth and decreasing gradually in thickness so as to enable the smoothing out what would otherwise have been an abrupt step in the first conductive level which could have caused weaknesses and discontinuities in the subsequent second level. Accordingly care is required to select a method for formation of this insulating layer which enables such smoothly sloping fillets to result from the anisotropic etching and in the examples specifically described with reference to the Figures of US-A-4594606 the second insulating layer is formed by a "painting" method, for example by mixing silicon hydroxide powder with alcohol to form a "solution" which is then coated onto the first metal layer and the substrate heated to evaporate off the alcohol. Although the method described in US-A-4594606 reduces the slope of the side walls bounding the contact area, the periphery of the window still provides a near vertical step over which the second conductive level has to be deposited and at which the aforementioned weaknesses and discontinuities in the second conductive level may occur. Furthermore, although, provided a suitable method for forming the insulating layer is used, the fillets or oxide spacers on the side walls bounding the contact area reduce the steepness of the side walls for a given window size, the fillets also decrease the area of the surface beneath the first conductive level exposed by the window and so it becomes difficult to ensure that the contact window is filled without resorting to the use of the commonly used but expensive technique of depositing a tungsten layer in the contact window. IBM Technical Disclosure Bulletin Vol 21 of August 1978 has at pages 1052 and 1053 an article concerned with the topography and consequent edge coverage problems which arise when covering a metal track with silicon dioxide using a sputtering process. More specifically this document is concerned with the problems presented by the small gaps in the insulator adjacent the side walls of the metal which result from the fact that, because the sputtering material is incident on the surface at a slight angle to the side walls of the metal, a geometric shadowing effect results causing small gaps in the insulator adjacent the metal track. In order to reduce step coverage problems, this document proposes the application of liquid polyimide which flows during its cure cycle to fill the gaps and provide some surface planarisation and points out that the characteristics of polyimide such as its water permeability do not present problems because the polyimide is in all cases covered by silicon dioxide or metal. According to one aspect of the invention, there is provided a method of forming an interconnection between conductive levels, which method comprises depositing and etching conductive material to define a first conductive level on a surface of a substrate body so that the first conductive level has a contact area, providing insulating material on the surface of the body to cover the first conductive level, exposing the contact area by anisotropically etching the insulating material to form a window in the insulating material larger than the contact area so that there is a gap between the periphery of the window and a side wall of the first conductive level bounding the contact area, the anisotropic etching leaving portions of the insulating material on the said side wall thereby reducing the gap, and depositing and then etching conductive material to define a second conductive level on top of the insulating material so that part of the second conductive level contacts the contact area within the window, characterised by providing a conductive etch stop layer resistant to attack during the etching of the second conductive level on the first conductive level prior to providing the insulating material over the first conductive level, depositing the insulating material to follow the underlying surface then, after anisotropically etching the insulating material to leave the portions on the said side wall, applying a flowable smoothing material over the insulating material so that the flowable material flows to fill the gap and causing the flowable material to solidify to provide a smooth surface between the periphery of the window and the portions of insulating material remaining on the side wall, and then etching the smoothing material to expose a top surface of the contact area prior to depositing and etching the conductive material to define the second conductive level, the insulating material being more resistant to attack during the etching of the conductive material to define the second conductive level than the smoothing material so that the remaining portions of the insulating material protect the side walls of the first conductive level against exposure during the etching of the second conductive level. In a second aspect, the present invention provides a method of manufacturing a semiconductor device, which method comprises providing depositing and etching conductive material to define a first conductive level on a surface of a semiconductor body so that the first conductive level has a contact area, providing insulating material on The surface of the body to cover the first conductive level, exposing the contact area by anisotropically etching the insulating material to form a window in the insulating material larger than the contact area so that there is a gap between the periphery of the window and a side wall of the first conductive level bounding the contact area, the anisotropic etching leaving portions of the insulating material on the said side wall thereby reducing the gap, and depositing and etching conductive material to define a second conductive level on top of the insulating material so that part of the second conductive level contacts the contact area within the window characterised by providing a conductive etch stop layer resistant to attack during the etching of the second conductive level on the first conductive level prior to providing the insulating material over the first conductive level, depositing the insulating material to follow the underlying surface then, after anisotropically etching the insulating material to leave the portions on the said side wall, applying a flowable smoothing material over the insulating material so that the flowable material flows to fill the gap and causing the flowable material to solidify to provide a smooth surface between the periphery of the window and the portions of insulating material remaining on the side wall, and then etching the smoothing material to expose a top surface of the contact area prior to depositing and etching the conductive material to define the second conductive level, the insulating material being more resistant to attack during the etching of the conductive material to define the second conductive level than the smoothing material so that the remaining portions of the insulating material protect the side walls of the first conductive level against exposure during the etching of the second conductive level. In a preferred embodiment, the method comprises providing the first conductive level as one or more conductive strips so that the contact area is on the or a conductive strip and is bounded by the elongate side walls of the strip, opening the window so that the window extends transversely over and is wider than the conductive strip and there is a gap between each elongate edge of the conducive strip and the periphery of the window, and providing the second conductive level as one or more conductive strips extending transversely over the conductive strip(s) of the first conductive level. A method embodying the invention thus enables the second conductive level to be deposited onto a surface which has been flattened or made more smooth by at least reducing the height of steps on the surface, so allowing the possibility of weakness or breaks in the second conductive level because of steep steps in the underlying surface to be reduced whilst allowing a contact window wider than the underlying contact area to be used. Moreover, the portions of tie insulating material remaining on the side wall of the first conductive level protect the first conductive level from attack during the etching of the conductive material to form the second conductive level. Furthermore, a method embodying the invention enables problems with regard to the filling of the contact window to be overcome or at least reduced as material can be used to fill the contact window other than the second conductive level. Also, as a method embodying the invention should result in a relatively flat second conductive level, it should help alleviate problems of pattern shift where compressive stresses applied by the subsequent plastic encapsulation may cause cracks in the final insulating layer, for example a plasma nitride layer, provided over the second conductive level and in the underlying insulating layers because it is thought that such problems arise or are at least exacerbated by stepped surfaces that enable the enveloping plastic material to get a good grip on the surface and thus removing or reducing such steps should alleviate these problems. A layer of insulating material such as a dielectric may be provided on the surface of the body prior to the first conductive level. The first conductive level may comprise aluminium or another conductive material such as doped polycrystalline silicon. The second conductive level may comprise aluminium, and may be, for example, an AlSi or AlCu layer. The conductive etch-stop layer, for example a layer of a titanium-tungsten alloy, provided on the first conductive level provides additional protection for the first conductive level during etching of the conductive level to define the second conductive level. An insulating material dispersed in a solvent may be used as the medium which is flowable and then solidifiable. Preferably, the medium used is a spin-on-glass which, as it is applied as a liquid, flows preferentially to lower levels so enabling the gap(s) to be filled whilst providing only a relatively thin covering layer of insulating material on the contact area, which relatively thin layer can be etched away relatively quickly and easily. The spin-on-glass should also enter niches or crevices in the insulating layer and thus reduce the height of or smooth any steps in the insulating layer so providing a smoother surface on which to provide the second conductive level avoiding or at least reducing the possibilities of weaknesses or undesired discontinuities in the second conductive level because of bad coverage on near vertical edges or steps and also avoiding or reducing the possibilities of undesired shorts between parts of the second conductive level which may otherwise occur if conductive material is left in steps or niches of the insulating layer after patterning of deposited conductive material to provide the desired second conductive level. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic top plan view of part of a semiconductor device illustrating an interconnection between conductive levels formed using a method embodying the invention; and Figures 2 to 6 are schematic cross-sectional views illustrating, in sequence, various steps in a method embodying the invention, Figures 2a, 3a, 4a, 5a and 6a being cross-sectional views taken along the line a-a in Figure 1 and Figures 2b, 3b, 4b, 5b and 6b cross-sectional views taken along the line b-b in Figure 1. Referring now to the drawings, it should of course be understood that the Figures are not to scale and that relative dimensions may have been altered in the interests of clarity in the drawings. Figure 1, 6a and 6b show part of a semiconductor body 1, in this example a monocrystalline silicon body, having double level metallisation. Although not shown, it will be appreciated that the semiconductor body will have doped regions forming semiconductor devices (such as, for example, bipolar transistors or insulated gate field effect transistors) for which the metallisation provides connections. As shown, a surface 2 of the semiconductor body is covered by an insulating layer 3 of silicon dioxide on top of which is provided a first conductive or metallisation level 4. The first conductive level 4 is, in this example, provided in the form of parallel aluminium tracks or strips 5 of which only two are shown, although there may be many more. A further layer 6 of insulating material is provided on top of the first conductive level 4 and the second metallisation level 7, in this example in the form of aluminium tracks or strips 8 extending transversely, as shown perpendicularly, of the strips 5 is provided on the insulating layer 6 after forming a window or contact opening 9 in the insulating layer 6 to expose a contact area 10 of the first and second metallisation levels 4 and 7. As can be seen from Figures 1, 3a and 3b especially, the contact area 10 (shown cross-hatched in Figure 1 in the interests of clarity) is, in this example, rectangular and is bounded in one direction by two opposed elongate side walls 5a of the strip 5. Again, although only two strips 8 are shown in Figure 1, there may be many more and there may be more than one interconnection between the first and second metallisation levels 4 and 7. A method of forming the first and second metallisation levels 4 and 7 and an interconnection between one strip 5 of the first metallisation level 4 and a strip 8 of the second metallisation level 7 which lies over the one strip 5 will now be described with reference to Figures 2 to 6. As shown in Figures 2a and 2b, after the layer 3 of thermally grown silicon dioxide has been provided on the surface 2 of the semiconductor body 1 and, if required, openings (not shown) formed in the layer 3 using conventional photolithographic and etching techniques to enable the subsequent metallisation to contact device regions of the semiconductor body 1, a layer 4a of aluminium is deposited onto the layer 3 to a thickness of, in this example, 500nm (nanometres) followed by a layer 4b of tungsten-titanium alloy to a thickness of, in this example, 100nm. The composite layer 4 is then patterned using conventional photolithographic and etching techniques to provide the conductive strips 5 of the first metallisation level 4. As will be appreciated, each strip 5 is thus a composite strip comprising the lower aluminium layer 4a and the upper alloy layer 4b. The strips 5 may, in this example, be 1.8 micrometres wide and may have a separation of 1.8 micrometres. Insulating material in the form of silicon dioxide is then deposited using a low temperature or plasma deposition technique so as to provide the insulating layer 6 covering the first metallisation level 4. In this example the insulating layer 6 is of the same thickness, that is 600nm, as the thickness of the layers 4a and 4b. Using conventional techniques, a photoresist layer (not shown) is provided on the insulating layer 6 and patterned to provide a window in the resist over the desired contact area 10 of the strip 5. The insulating layer 6 is then etched anisotropically, for example by reactive ion etching, to form, as shown in Figures 3a and 3b, the contact window or opening 9. The dimensions of the contact opening 9 to be formed in the insulating layer 6 are, of course, determined by the selected dimensions of the window in the resist (not shown). The window in the resist is so dimensioned that the contact window 9 extends beyond each side wall 5a for a distance somewhat greater than the thickness Tox of the insulating layer 6 and the anisotropic etching through the resist window is controlled so as to stop at the tungsten-titanium alloy layer 4b which thus acts as an etch-stop barrier. This leaves fillets 6b of the insulating material, in this example silicon dioxide, on the side walls 5a of the strip 5. The precise width w of the contact opening 9 will be determined by the particular processing conditions and tolerances but may, where, as described above, the thickness Tox of the passivating layer 6 is 0.6 micrometres and the width of the strip 5 is 1.8 micrometres, be 4 micrometres. The length l of the contact opening 9 will be determined, as will be described below, by the desired dimension of the contact area 10 and of course taking into account processing conditions and tolerances. The opening of tie contact window 9 to expose the contact area 10 provides, in the direction of the width w of the contact widow 9, a respective gap 11 in the insulating layer 6 on each side of the contact area 10, each of the two gaps being defined between a respective side wall 9b of the periphery of the contact window 9 and the adjacent fillet 6b. Before the second conductive level 7 is provided, material is provided in the gaps 11 to planarise, that is to smooth or flatten the surface between the periphery of the window and the contact area. In this example, the material is a planarising medium which is initially flowable and so can be applied to the surface so as to enter any cracks or crevices and thus fill the gaps 11 and which can then be set or solidified. Preferably the medium is a spin-on-glass which when heated, after application to the surface, to evaporate the carrier solvent solidifies or sets to form a glassy layer of silicon dioxide. There are many different types of spin-on-glass which may be suitable for present purposes. Thus, for example, thee spin-on-glass may comprise phenylsiloxane in 1-propanol as in the case of, for example, the spin-on-glass produced by Allied Chemical Co. of the United States of America under the trade name Accuglass 204, or methylphenylsiloxane in 1-propanol as in the case of, for example, the spin-on-glass produced by Allied Chemical Co. under the trade name Accuglass 108 or phosphorus doped silanol in ethanol as in the case of, for example, the spin-on-glass produced by Tokyo Otika Kogyo Co. of Japan under the trade name P48340 or any other suitable spin-on-glass. The spin-an-glass is applied to the surface of the semiconductor body 1 whilst the semiconductor body or wafer 1 is rotated so as to spread the spin-on-glass solution evenly. The amount of solution applied is, for example, selected to be that which would produce a layer 50 to 100nm thick on a flat test wafer of the same surface area. The spin-on-glass thus flows over the surface entering the crevices and niches so as to flatten or smooth the surface and reduce the height of any steps in the surface. The spin-on-glass flows preferentially to the lowest level on the surface and so will preferentially fill the gaps 11 whilst providing a thinner covering on relatively higher surface areas. The degree of planarisation or smoothing of the surface, that is the amount by which any steps in the surface are reduced, may be controlled by controlling the concentration of the silica medium, for example methylsiloxane, in the alcohol solvent. The semiconductor body 1 is then heated to evaporate off the alcohol solvent in the spin-on-glass and so leave a glassy electrically insulating layer 13 of silicon oxide 13. As will be appreciated from the above, because of the initial flowable properties of the spin-on-glass, the layer 13 should be, as shown in Figures 4a and 4b, thinnest on top of the steps in the passivating layer 6, that is on the top surfaces 6′ of the highest or thickest regions and thickest within the contact window 9 and in the niches or crevices 6′′ defined by the steps in the passivating layer. This thickness of the layer 13 on top of the contact area 10 of the strip should be intermediate these two extremes. The layer 13 is then etched back uniformly over the surface of the semiconductor body 1 using any suitable conventional wet or dry etching tehnique until the top surface of the alloy layer 4b is exposed. As shown in Figures 5a and 5b, the gaps 11 and any other crevices or steps in the surface remain filled with the insulating material 13b formed by the solidified spin-on-glass. Fillets 13a of the solidified spin-on-glass are also left on the elongate wide walls 9a of the periphery of the contact window 9, that is the side walls extending across the strip 5. The width of the contact area 10 across the strip 5 is thus determined by the side walls 5a of the strip 5 whilst the length of the contact area 10 along the strip 5 is determined by the length 1 of the contact window 9 and by the fillets 13a. Next, as illustrated in Figures 6a and 6b, a second aluminium-containing conductive layer, for example AlSi or AlCu, and in this example, 1000nm in thickness, is deposited on the planarised insulating layer. Using conventional photolithographic and etching techniques, the second conductive layer is patterned to define the second conducive level 7. As indicated above, in this example, the second conductive level 7 comrises parallel strips or tracks 8 extending transversely, as shown perpendicularly, of the strips or tracks 5. The strips or tracks 8 may have a width a' of 1.8 micrometres and may be separated by the same distance. During the etching to pattern the second aluminium layer, there is a danger of further undesired etching the first aluminium layer. However, in the present arrangement, this possible undesired etching may be prevented or at least reduced because the alloy layer 4b protects the top surface of the contact area 10 whilst the fillets 6b serve to protect the edges of the strip 5 from being eroded during the etching, the dielectric material, in this example silicon dioxide, of the insulating layer 6 being less susceptible to attack by the aluminium etchant than the remaining solidified spin-on-glass. The spin-on-glass also serves, as indicated above, to smooth or flatten other steps or crevices in the passivating layer. Thus, for example as shown in Figures 5a to 6b, the niches 6'' in the insulating layer 6 are filled with solidified spin-on-glass 13c so reducing the height of or smoothing out the step in the surface. Thus a smoother surface is provided onto which the second conducive level is deposited so reducing the possibilities of the weaknesses or discontinuities which may otherwise occur in the aluminium track if there were sharp discontinuities and vertical edges in the surface on which the aluminium is deposited. Also, as will be appreciated, it can be extremely difficult to ensure that an etchant removes material from crevices or niches such as the niche 6'' and accordingly where such crevices or niches exist, undesired aluminium may remain after patterning of the second conductive level and such undesired aluminium may cause shorting of adjacent strips 8. The fact that the niches 6'' are filled or at least flattened out by the solidified spin-on-glass 13 should at least reduce such problems. As shown in Figures 6a and 6b, the strip 8 overlying the one strip 5 thus interconnects with or contacts with the underlying contact area 10 of the one strip 5 via the contact window 9. Although, materials other than flowable and then solidifiable or settable materials such as spin-on-glass would be used to fill the gaps 11, for example a low temperature silicon oxide could be deposited onto the surface so as to fill the gaps 11 and then etched back to expose the alloy layer 4b, these would not have the covering and crack-filling properties associated with a flowable material like spin-on-glass. In the arrangement described above, the insulating layer 6 is etched anisotropically to leave the fillets 6b on the side walls 5a. If, when opening the contact window 9, the insulating layer 6 were etched, for example isotropically, so that no fillets are left the side walls 5a would be exposed. However, as indicated above, the fillets 6b assist in protecting the side walls 5a from attack by the etchant during patterning of the second conductive level. The fillets 6b also reduce, for a given size of contact window 9, the volume of the gaps 11 required to be filled by the spin-on-glass and the spin-on-glass should fill such smaller gaps more evenly and consistently than larger gaps. Similarly the first and second conductive levels need not necessarily be in the form of parallel strips but could each adopt any desired conductive pattern. Furthermore, the conductive levels may be formed of other conductive materials or laminates of two or more different conductive materials, for example the first conductive level could be a doped polycrystalline silicon level. The first conductive level could in some circumstances ey provided directly on the surface of the substrate body. Also the method may be applied where one or more further conductive levels are provided over the second conductive level in a similar manner to that in which the second conductive level is provided over the first conductive level. A method embodying the invention may be used where the substrate body is other then monocrystalline silicon, for example where the substrate body comprises one or more different semiconductor materials, such as III-V compounds. Also, a method embodying the invention may be used where the substrate body is other than a semiconductor body, for example in liquid crystal technology.

Claims: A method of forming an interconnection between conductive levels, which method comprises depositing and etching conductive material to define a first conductive level on a surface of a substrate body so that the first conductive level has a contact area, providing insulating material on the surface of the body to cover the first conductive level, exposing the contact area by anisotropically etching the insulating material to form a window in the insulating material larger than the contact area so that there is a gap between the periphery of the window and a side wall of the first conductive level bounding the contact area, the anisotropic etching leaving portions of the insulating material on the said side wall thereby reducing the gap, and depositing and then etching conductive material to define a second conductive level on top of the insulating material so that part of the second conductive level contacts the contact area within the window, characterised by providing a conductive etch stop layer resistant to attack during the etching of the second conductive level on the first conductive level prior to providing the insulating material over the first conductive level, depositing the insulating material to follow the underlying surface then, after anisotropically etching the insulating material to leave the portions on the said side wall, applying a flowable smoothing material over the insulating material so that the flowable material to flow to fill the gap and causing the flowable material to solidify to provide a smooth surface between the periphery of the window and the portions of insulating material remaining on the side wall, and then etching the smoothing material to expose a top surface of the contact area prior to depositing and etching the conductive material to define the second conductive level, the insulating material being more resistant to attack during the etching of the conductive material to define the second conductive level than the smoothing material so that the remaining portions of the insulating material protect the side walls of the first conductive level against exposure during the etching of the second conductive level. A method of manufacturing a semiconductor device, which method comprises providing depositing and etching conductive material to define a first conductive level on a surface of a semiconductor body so that the first conductive level has a contact area, providing insulating material on the surface of the body to cover the first conductive level, exposing the contact area by anisotropically etching the insulating material to form a window in the insulating material larger than the contact area so that there is a gap between the periphery of the window and a side wall of the first conductive level bounding the contact area, the anisotropic etching leaving portions of the insulating material on the said side wall thereby reducing the gap, and depositing and etching conductive material to define a second conductive level on top of the insulating material so that part of the second conductive level contacts the contact area within the window, characterised by providing a conductive etch stop layer resistant to attack during the etching of the second conductive level on the first conductive level prior to providing the insulating material over the first conductive level, depositing the insulating material to follow the underlying surface then, after anisotropically etching the insulating material to leave the portions on the said side wall, applying a flowable smoothing material over the insulating material so that the flowable material flows to fill the gap and causing the flowable material to solidify to provide a smooth surface between the periphery of the window and the portions of insulating material remaining on the side wall, and then etching the smoothing material to expose a top surface of the contact area prior to depositing and etching the conductive material to define the second conductive level, the insulating material being more resistant to attack during the etching of the conductive material to define the second conductive level than the smoothing material so that the remaining portions of the insulating material protect the side walls of the first conductive level against exposure during the etching of the second conductive level. A method according to Claim 1 or 2, which comprises providing the first insulating material by depositing silicon oxide using a low temperature or plasma deposition technique. A method according to Claim 1, 2 or 3, which comprises providing the first conductive level as one or more conductive strips so that the contact area is on the or a conductive strip and is bounded by the elongate side walls of the strip, opening the window so that the window extends transversely over and is wider than the conductive strip and there is a gap between each elongate edge of the conductive strip and the periphery of the window, and providing the second conductive level as one or more conductive strips extending transversely over the conductive strip(s) of the first conductive level. A method according to Claim 1, 2, 3 or 4, which comprises using as the flowable and then solidifiable medium insulating material dispersed in a solvent and which comprises solidifying the medium by evaporating off the solvent. A method according to Claim 5, which comprises using spin-on-glass as the flowable then solidifiable medium. A method according to any one of the preceding claims, which comprises using a tungsten-titanium alloy as the etch stop layer.

Inventors: GOOTZEN WILHELMUS F M PHILIPS, OSINSKI KAZIMIERZ C O PHILIPS, GOOTZEN, WILHELMUS F.M. PHILIPS INTERNATIONAL B.V., OSINSKI, KAZIMIERZ C/O PHILIPS INTERNATIONAL B.V.
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01L21
IPC codes: H01L21, H01L23