Method of forming conductors within an insulating substrate

Abstract: A method for forming an electrically conductive line between two layers of insulating material and method for connecting the line through both layers of the insulating material to the opposite surfaces is provided. In the method, first, second and third layers (10, 20, 28) of insulating material are provided wherein said first and third layers are separated by said second layer of insulating material which is different in etch rate from the first and third layers. The edge portion of all three layers is exposed and the insulating layer of the second material is selectively etched to remove the revealed edge portion and provide a slot (36) between the first (10) and third (28) layers of insulating material. Also openings (38) are provided in both the first and third layers of insulating material which communicate with the slot and extend respectively through the layers of the first and third insulating material. Thereafter, a conductive material such as tungsten is deposited in the slot and the openings and also on the face of the stacked insulating material. Finally, the excess tungsten is removed from the faces of the insulating material of the first and third layers leaving a conductive line sandwiched between the first and third insulating layers of the material; also metal remains in the openings formed to thereby form conductive studs extending from the line to the opposite surfaces of the insulating material sandwich so formed.

Description: This invention relates generally to the formation of metal lines embedded in substrates and their interconnections through the substrates, and more particularly to a method for forming metal conductors imbedded within the substrate and which conductors have metal connections extending to the opposite surfaces of the substrate. In even more particular aspects, this invention relates to a method of forming a metal line between two layers of insulating material which form a substrate or a portion of the substrate and concomitantly forming metal studs which extend through the layers of the substrate material to thereby provide electrical interconnections from either side of the substrate to the metal connection line embedded within the substrate. As the dimensions of the ULSI circuitry becomes smaller and smaller with the advancement of technology, it becomes increasingly necessary to form smaller metal conductors within an insulating substrate and provide precise connections to surfaces on both side of the insulating substrate all within a very small and closely controlled dimensions. Also, technology is advancing such that electrical devices can be formed on side walls of trenches, which devices must be wired, or connected. In the past, it has been conventional practice to form the substrate in several different layers depositing the metal layers between the insulating materials and then etching the exposed metal to the desired pattern. The etched pattern is connected to various levels by means of metal fill vias formed through the insulating layers. While this does work quite well in many instances, it is difficult to obtain very close tolerances with small lines requiring precision interconnection to the opposite surfaces. Further, this technique cannot be readily implemented in side wall electrical device technology. Thus, the invention as claimed is intended to remedy these drawbacks. According to claim 1 of the present invention a method of providing electrical conductive material between two layers of insulating materials is provided. Further, claim 8 discloses a multi-layer substrate according to the present invention. The method comprises the steps of providing first, second and third layers of insulating material wherein the second layer of insulating material is interposed between said first and third layers. The second layer has an etch rate which differs from that of said first and third layers and which can be selectively removed without removing the material of the first and third layers. At least one edge portion of the stacked layers of material is exposed. Thereafter an edge portion of the second material is selectively removed from the exposed portion to provide a slot or undercut between the first and third layers of insulating material. Thereafter a conductive material is deposited in said slot and around the exposed edge portions of the first and third layers of insulating materials. Finally the conductive material laying outside of the slot around the edge portion of the remaining insulating material is removed to thereby provide a conductive line between two layers of insulating material. For a better understanding of the present invention, together with further objects and advantages in the following preferred embodiments of the invention are described with reference to the accompanying drawings, in which: Figs. 1a through 1j depict, somewhat diagrammatically, sequentially the steps of the preferred embodiment of the present invention; and Figs. 2a through 2e depict a somewhat modified version of certain of the steps of the invention from those of Figs. 1a through 1h. The invention can best be understood by referring to the drawings which depict sequentially the steps involved in the present invention. As shown in Figure 1a, a layer of insulating material 10 preferably silicon dioxide, is mounted or deposited on a base layer 12. The deposit is preferably by chemical vapor deposition (CVD). One preferred technique is performed in a parallel plate reactor using 2% silane (SiH4) with a carrier of He, with an N₂0 flow of .75 liters/min. at a pressure of 252.7 Pa (1.9 Torr) at 300°C, the upper electrode being biased. The base layer preferably is a silicon substrate which has an etch stop 13 such as Al₂O₃ coated thereon, by conventional methods such as reactive sputtering. A layer of photoresist material 14 is applied on to the insulating layer 10. Any conventional photoresist material may be used; and, by conventional photolithographic techniques, the photoresist material 14 is exposed and developed to reveal the underlying substrate 10 in the pattern as shown in Figure 1b. The structure is then etched to remove the exposed silicon dioxide in the layer 10, the remaining photoresist material acting as an etch mask. The etching is preferably done by reactive ion etching techniques in CHF₃ + CO₂ or CF₄ at a 13.3 Pa (100 Millitorr) or less. The etching continues until all the exposed material has been removed down to the stop 13. Following this the remaining photoresist material is removed with the resulting structure being as shown in Figure 1c in which a trough or trench 16 has been etched into the silicon with groves 18 extending orthogonally therefrom. At this point a second layer of a second insulating material 20 is blanket deposited over the first insulating layer 10 and the exposed portion of the underlying stop 13 and preferably is planarized. It is essential that the second insulating material be of a different composition or structure from the first insulating material or in some other way differs such that it will react with an etch media which is essentially unreactive or much less reactive to the first insulating material. Expressed another way, the material 20 must have an etch rate in some etch medium which differs from that of the material 10. In the preferred embodiment this second insulating material 20 is silicon nitride Si₃N₄. A preferred method is utilizing plasma enhanced chemical vapor deposition (PECVD). Preferably it is done in Silane (SiH₄) and ammonia (NH₃) with 175 SCCM of Silane and 325 SCCM of NH₃, a presence of 266 Pa (2 Torr) and at 375°C, with a power of 175 watts. A photoresist material 22 is then applied over the insulating material 20 and exposed and developed to reveal the underlying insulating material in the layer 20 as shown in Figure 1d. It will be noted in this embodiment that the edge of the remaining photoresist material, which will act as an etch mask, on the left side 24 is coincident with the edge of the insulating material 10 whereas the edge of the remaining photoresist material on the right side is somewhat offset with the edge of the insulating material 10 on its right side where the trough 16 is forming a step 26 as shown in Figure 1e. At this point, the silicon nitride is etched anisotopically preferably by reactive ion etching in CHF₃+CO₂ at a low pressure, e.g. 4 Pa (30 Millitorr) or less. This will selectively anisotopically etch the silicon nitride but will be essentially unreactive with the silicon dioxide in the insulating layer 10. The resulting structure is as shown in Figure 1e. At this point the remaining photoresist material 22 is removed and a third insulating material 28 which must have a different etch rate from that of the insulating material in the second layer and preferably is the same as the insulating material of the first layer, i.e. Si0₂ is deposited uniformally over the insulating layers 10 and 20 completely filling the trough 16. It should be noted at this point that the groves 18 in insulating layer 10 are filled with the silicon nitride material of the insulating layer 20. A layer of photoresist 30 is applied on top of the third insulating layer 28 and exposed and developed to reveal the underlying silicon dioxide in the insulating layer 28 as shown in Figure 1f. It should be noted that the right side of the remaining photoresist material coincides with the right side of the insulating layer 10 and overhangs the right side of the insulating layer 20 and the left hand side of the exposed photoresist is coincident with the left hand sides of both the insulating layer 10 and the second insulating layer 20. This is shown in Figure 1f. The revealed silicon dioxide of the insulating layer 28 is then etched preferably by reactive ion etching techniques as previously described in CHF₃ + O₂ at 26.6 Pa (200 Millitorr) or less which will result in the structure shown in Figure 1g. In this structure groves 32 are formed in layer 28 which extend orthagonally with respect to the central trough 16. The remaining photoresist material is then removed. Following the removal of the photoresist material 30, the silicon nitride insulating layer 20 is isotopically etched by any conventional technique which will etch the silicon nitride but not the Si0₂. A preferred technique is to utilize hot phosphoric acid (H₃PO₄). This will undercut the silicon nitride layer forming a longitudinally extending slot 33 as shown in Figure 1h. This will also remove the silicon nitride in slots 18, also as shown in Figure 1h. It should be noted that this slot 33 is formed only on the left hand side of the structure where the silicon nitride is exposed and not on the right side where the layer 28 of the silicon dioxide covers the edge of the silicon nitride in the insulating layer 20. At this point a layer of metal 34 is conformably deposited as shown in Figure 1i. Preferably this metal is tungsten which is blanket deposited by conventional CVD techniques. A preferred method of blanket depositing the tungsten is to first deposit a seed layer about 50 µm (500 angstroms) thick of tungsten silicide (WSi₂) in a chemical vapor deposition process utilizing WF₆ H₂ and SiH₄ gasses at a pressure of about 20 Pa (150 Millitorr) and a temperature of about 450°C. Following the deposition of the seed layer a tungsten metal is deposited also utilizing CVD techniques in WF₆ H₂ and SiH₄ gasses at about 20 Pa (150 Millitorr) and 450°C. These depositions can take place in a coldwall reactor, such technique for deposition of tungsten being well known in the art. Finally, the tungsten is subjected to a wet chemical etch of dilute peroxides which stops on the seed layer of WSi₂. The seed layer is etched in a 20/1 ratio of nitric acid and ammonium fluoride. This etch will not etch tungsten or oxides. These combined etches will remove the tungsten and WSi₂ on the exposed edges of the stack of insulating layers 10 and 28, and the etching is stopped when only the tungsten and WSi₂ which is deposited in the slot 33 and in the groves 18 and 32 still remains. This final structure is shown in Figure 1j, the resulting structure being an electrical line 36 disposed between insulating layers 10 and 28 with upwardly extending studs 38 to form electrical connections extending through openings in the insulating layer 28 and downwardly extending studs 40 to form electrical connections through openings in the insulating layer 10. At this point, the basic wiring plan is established and the structure so formed can be utilized for various wiring schemes on various devices. For example, this technique is particularly useful if one wishes to have sidewall formed semiconductor devices, since it provides a very viable and well designed technique for connecting to sidewall formed devices. Further it should be understood that there are many modifications of the very specific technique which has been described. For example, if one wished to have wiring on both sides of the opening then the right side of the opening could be formed similarly to the left side which would provide wiring on that side as well. Further, if it is anticipated or determined that there are or may be problems controlling the etching of the silicon nitride layer isotopically by the hot phosphoric acid to form a uniform undercut as shown in Figure 1h, then the process can be slightly modified as follows: In the step of applying the photoresist 22 over the silicon nitride 20 as shown in Figure 1d, the photoresist is applied in a band or strip about one micron wide or whatever size is desired for the final electrical line width as shown in Figure 2a. Hence, when the anisotropic etch in CHF₃+CO₂ is performed, the exposed silicon nitride to the left of the photoresist 22 is also removed as well as that in the trough 16 leaving a silicon nitride band 20 of about 1 micron in width below the photoresist 22 as shown in Figure 2b.In the subsequent steps the photoresist 22 is removed, and the silicon dioxide 28 is deposited as previously described, and the photoresist 30 is applied thereon and patterned as previously described, as shown in Figures 2c, and the SiO₂ 28 etched to form the structure shown in Figure 2d. As shown in Figure 2c through 2e, the layer of silicon dioxide 28 also fills in to the left of the silicon nitride layer 20; thus, when the hot phosphoric acid isotopically etches the silicon nitride 20 to produce the structure shown in Figure 2e, the etching will automatically be stopped when all of the silicon nitride has been etched to reveal the layer of silicon dioxide 28 adjacent the silicon nitride. The metal can then be deposited and etched in the same manner as described in the previous embodiment and shown in Figures 1i and 1j.

Claims: A method of providing an electrically conductive material between two layers of insulating material both deposited on a substrate (12), in superposed relationship, said method comprising the steps of; providing first (10), second (20) and third (28) layers of insulating material forming a stack on said substrate (12), wherein said first (10) and third (28) layers are separated by said second (20) layer, said second layer (20) having an etch rate, with respect to one etch medium, that differs from that of said first and third layers, exposing at least one edge portion of said first (10), second (20) and third (28) layers of insulating material, selectively removing the exposed edge portion of said second insulating material (20) to provide a longitudinal slot (33) between said first (10) and third (28) layers of insulating material, thereafter depositing conductive material (34) in said slot and around said exposed edge portions of the first (10) and third (28) layers of insulating materials, and removing the conductive material laying outside said slot on said exposed edge portions. The method as defined in claim 1 further comprising forming at least one opening (18, 32) through at least one of said first (10) and third (28) layers of said insulating material, each of said openings (18, 32) intersecting said slot (33), and depositing conductive material (34) in each opening so formed. The method as defined in claim 2 wherein said openings (18,32) are formed through both first (10) and third (28) layers of insulating material. The method as defined in any one of the preceding claims wherein said first (10) and third (28) insulating layers are the same material, preferably silicon dioxide. The method as defined in claim 4 wherein said second (20) insulating material is silicon nitride. The method as defined in any one of the preceding claims wherein the conductive material (34) is tungsten. The method as defined in any one of the preceding claims wherein the third layer (28) of insulating material also has a portion thereof deposited onto said first layer (10) of insulating material and lying adjacent said second layer (20) of insulating material; whereby said portion of said third insulating material (28) on said first insulating material (10) acts as a stop when removing said second insulating material (20) form between said first and third layers of insulating materials. A multi-layer substrate for mounting electrical devices on at least one side thereof comprising, first (10) and second (28) layers of dielectric material in superposed relationship deposited on a substrate (12), a layer (34) of electrically conductive material interposed between said first and second layers of dielectric material, said layer of conductive material having at least two studs (38, 40) extending through at least one of said layers of said dielectric material at spaced locations to provide an electrical path between said studs through said electrical conductive material, said layer of conductive material and said studs being a single unitary mass of material, free of interfaces and connections there between. The substrate as defined in claim 8 where at least one stud (40) extends through said first layer of dielectric material (10) and at least one other stud (38) extends through said third layer of dielectric material (28). The substrate as definedd in claim 8 wherein two studs extend through the same layer of dielectric material at spaced locations.

Inventors: CRONIN JOHN EDWARD, KAANTA CARTER WELLING, LEACH MICHAEL ALBERT, CRONIN, JOHN EDWARD, KAANTA, CARTER WELLING, LEACH, MICHAEL ALBERT
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H01L21
IPC codes: H05K3, H01L21, H01L27