Low energy passivation of conductive material in damascene process for semiconductors

[0002] The present invention relates generally to semiconductors and more specifically to a manufacturing method for dual damascene semiconductors.

[0003] In the process of manufacturing integrated circuits, after the individual devices such as the transistors have been fabricated in the silicon substrate, they must be connected together to perform the desired circuit functions. This connection process is generally called “metalization” and is performed using a number of different photolithographic and deposition techniques.

[0004] One metalization process, which is called the “damascene” technique, starts with the placement of a first channel dielectric layer, which is a silicon dioxide or other oxide layer, over the semiconductor devices. A first damascene step photoresist is then placed over the dielectric layer and is photolithographically processed to form the pattern of the first channels. An anisotropic etch, generally an oxide etch, is then used to etch out the channel dielectric layer to form the first channel openings. The damascene step photoresist is then stripped and a conductive material is deposited in the first channel openings.

[0005] Some conductive materials, such as copper, require preparatory steps before deposition. An optional adhesion material, such as tantalum or titanium, is deposited followed by a barrier material, such as tantalum nitride or titanium nitride. The combination of the adhesion and barrier material is collectively referred to as “barrier layer” herein. The barrier layer is used to prevent failure causing diffusion of the conductive material of the channels into the dielectric layer and the semiconductor devices. A seed layer is then deposited on the barrier layer to form a conductive material base, or “seed”, for subsequent electro-deposition of the conductive material.

[0006] The conductive material deposited in the first channel openings is then subjected to a chemical-mechanical polishing process which removes the materials above the first channel dielectric layer and above the first channel openings. With the chemical-mechanical polishing, the conductive material is “damascened” into the first channel dielectric layer to form the first conductive channels. After the chemical-mechanical polishing process the conductive material in the channels is passivated by deposition of a thin dielectric layer. The thin dielectric layer protects the conductive material from oxidation and prevents the diffusion of the conductive material into subsequent dielectric layers.

[0007] For multiple layers of channels, the “dual damascene” technique is used in which the channels and vias are formed at the same time. In one example, the via formation step of the dual damascene technique starts with the deposition of a thin dielectric etch stop layer, such as a silicon nitride, over the first channels and the first channel dielectric layer. Subsequently, a via dielectric layer is deposited on the etch stop layer. This is followed by deposition of a thin via dielectric etch stop layer, generally another nitride layer. Then a via step photoresist is used in a photolithographic process to designate round via areas over the first channels.

[0008] A stop layer etch, generally a nitride etch, is then used to etch out the round via areas in the via nitride. The via step photoresist is then removed, or stripped. A second channel dielectric layer is then deposited over the via dielectric stop layer and the exposed via dielectric layer. A second damascene step photoresist is placed over the second channel dielectric layer and is photolithographically processed to form the pattern of the second channels. An anisotropic etch is then used to etch the second channel dielectric layer and the via dielectric layer to form the second channel openings and the via areas down to the thin etch stop layer above the first channels. The damascene photoresist is then removed, and a stop layer etch process removes the via etch stop layer above the first channels in the via areas.

[0009] For conductive materials such as copper as previously described, a barrier layer is then deposited to coat the via openings and the second channel openings. Next, a seed layer is deposited on the barrier layer. This is followed by a deposition of the conductive material in the second channel openings and the via openings to simultaneously fill the second channel and the vias. A second chemical-mechanical polishing process defines the second channel and leaves the two vertically separated channels connected by a cylindrical via. Again, after the chemical-mechanical polishing process the conductive material in the channels is passivated by deposition of a dielectric layer. The dielectric layer protects the conductive material from oxidation and prevents the diffusion of the conductive material into subsequent dielectric layers.

[0010] The use of the damascene techniques eliminates metal etch and dielectric gap fill steps typically used in the metalization process. The elimination of metal etch steps is important as the semiconductor industry moves from aluminum to other metalization materials, such as copper, which are very difficult to etch.

[0011] For conductive materials, such as copper, the protective dielectric layer is generally of silicon nitride. Unfortunately, with smaller and smaller semiconductor geometries, since the dielectric constant of silicon nitride is relatively high, it causes capacitance problems between the channels and between the interconnect layers.

[0012] The present invention provides a method for dry passivation of the surface of exposed conductive materials in semiconductor integrated circuits to protect from oxidation and prevent diffusion. After deposition of a conductive material, such as copper, and chemical-mechanical polishing, a low energy implantation process is used to nitride the conductive material.

[0013] The present invention provides a method for passivating the surface of exposed conductive materials in semiconductor integrated circuits without using a high dielectric constant protective layer such as SiN.

[0014] The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.

[0015] FIG. 1 (PRIOR ART) is a plan view of aligned channels of the prior art integrated circuit;

[0016] FIG. 2 (PRIOR ART) is a cross-section of FIG. 1 (PRIOR ART) along line 2—2; and

[0017] FIG. 3 is a cross-section similar to FIG. 2 (PRIOR ART) during an intermediate stage of the processing of the present invention.

[0018] Referring now to FIG. 1 (PRIOR ART), therein is shown a plan view of a prior art pair of perpendicularly aligned semiconductor channels of a conductive material, such as copper, disposed on a production integrated circuit 100 in a second channel dielectric layer 101. A first channel 102 is shown disposed below a second channel 104 which extends substantially perpendicular to the first channel 102 in the plan view. A cylindrical via 106 connects the first and second channels 102 and 104 and is a part of the second channel 104 by being deposited concurrently by a dual damascene technique.

[0019] Referring now to FIG. 2 (PRIOR ART), therein is shown a cross-section of FIG. 1 (PRIOR ART) along 2—2. The first channel 102 is disposed in the first channel dielectric layer 108 over a polysilicon gate 110 and a semiconductor device dielectric layer 112 of a semiconductor device (not shown). The first and second channels 102 and 104 are connected by the cylindrical via 106 and are in horizontal planes separated vertically by an etch stop layer 114, a via dielectric layer 116, and a thin via etch stop layer 118. Also shown disposed around the first channel 102 are a barrier layer 120 and seed layer 122, and around the second channel 104 and the cylindrical via 106 is a barrier layer 124 and seed layer 126. For simplicity, these layers are not shown in FIG. 1 (PRIOR ART).

[0020] For copper or copper alloy conductive materials, the barrier layers are made up of barrier materials such as tantalum, titanium, tungsten, their alloys, compounds thereof (especially their nitrides) and combinations thereof. The barrier materials are intended to prevent detrimental migration of copper into the dielectric materials. The copper seed layers are deposited on top of the barrier layers to provide a conductive layer for the subsequent electroplating of copper which makes up the channels.

[0021] After deposition of the conductive material which will fill the channels and cover the dielectric layers, the conductive material outside the channels must be removed. The removal is accomplished using a chemical-mechanical polishing process which abrades the conductive material from the surface of the dielectric layers until it is left only in the channels.

[0022] It should be understood that each layer of dielectric material and conductive material are subject to the chemical-mechanical polishing process to a smoothness of approximately 100 A° before the deposition of the subsequent dielectric layers. Thus, the first channel dielectric layer 108 and the first channel 102 are subject to the chemical-mechanical polishing process in preparation for the etch stop layer 114. And the second channel dielectric layer 101 and second channel 104 are subject to the chemical-mechanical polishing process in preparation for a protective layer 130 of silicon nitride.

[0023] Referring now to FIG. 3, therein is shown a cross-section similar to FIG. 2 (PRIOR ART) of an integrated circuit 200. For ease of understanding, the same elements as shown in FIG. 2 (PRIOR ART) are numbered with the same numbers. The first channel 102 is shown disposed in a first channel dielectric layer 108 over the polysilicon gate 110 and the semiconductor device dielectric layer 112, which are portions of a semiconductor device (not shown). Disposed vertically above the first channel 102 are the etch stop layer 114, the via dielectric layer 116, the thin via etch stop layer 118, and the second channel dielectric layer 101. Again, the dielectric layers are generally of silicon dioxide, and the etch stop layers are generally of silicon nitride (SiN). If copper is passivated, then etch stop layer 114 can be any material and not necessarily SiN.

[0024] Also shown disposed around the first channel 102 are the barrier layer 120 and the seed layer 122. In the preferred embodiment, the barrier layers are made up of barrier materials such as tantalum, titanium, tungsten, their alloys, compounds thereof (especially nitrides), and combinations thereof The channels are generally of copper, aluminum, polysilicon, gold, silver, alloys thereof, and combinations thereof.

[0025] Finally, a chemical-mechanical polishing step is used to remove all the material above the second channel dielectric layer 101 and to expose it for further processing.

[0026] In FIG. 3, the top surface 230 of the second channel 104 is shown passivated by a dry conductive material passivation process. For the conductive material copper, a low energy ion implantation of nitrogen is performed. The energy level is approximately 100 KeV. This would convert the surface copper to copper nitride (CuN). The copper nitride forms both an excellent oxidation barrier as well as an excellent diffusion barrier. Therefore, a deposited, passivated SiN layer (with a dielectric of 8.0) is not needed and the etch stop layer can be silicon oxynitride (SiON) or other material with a low dielectric constant compared to SiN. This greatly reduces interchannel capacitance problems.

[0027] In production, a conventional first damascene process is used to deposit the first channel 102 in the first channel dielectric layer 108 above portions of the polysilicon gate 110 and 112 on a production integrated circuit 200. The damascene process is a photolithographic process which uses a mask to define a first channel opening in the first channel dielectric layer 108. The first channel opening is then filled with the barrier layer 120 and the seed layer 122 which line the opening. A first conductive material, such as copper or a copper alloy, is deposited using conventional metal deposition techniques, such as physical vapor deposition, chemical vapor deposition, or a combination thereof, although electroplating is preferred, to form the first channel 102. The first channel 102 and the first channel dielectric 108 are subject to chemical-mechanical polishing. The surface 130 of the first channel 102 is then subject to the roughening process.

[0028] The etch stop layer 114, the via dielectric layer 116, and the via etch stop layer 118 are then successively deposited on top of the first channel 102 and the first channel dielectric layer 108 using conventional deposition techniques.

[0029] By using the via photoresist and the via photolithographic process followed by etching of the via etch stop layer 118, the basis for the cylindrical via 106 is formed. There is then a subsequent deposition of the second channel dielectric layer 101. The second damascene process uses a further mask to define the second channel opening in the second channel dielectric layer 101. Using an anisotropic dielectric etch, the second damascene process also forms the cylindrical via 106 down to the etch stop layer 114. The anisotropic dielectric etch etches faster in the vertical direction of FIG. 3 than in the horizontal direction. The etch of the etch stop layer 114 then exposes a portion of the first channel 102.

[0030] Next, the barrier layer 124 and the seed layer 126 are deposited to line the second channel opening by a process such as physical-vapor deposition or chemical-vapor deposition. The conductive material, generally copper or a copper alloy, is deposited by using the seed layer 126 as an electrode for electroplating copper ions from a plating bath. After the second channel opening is filled, a chemical-mechanical polishing process is used to polish the conductive material, the seed layer 126, and the barrier layer 124 down to the surface of the second channel dielectric layer 101. This damascenes the conductive material into the second channel dielectric layer 101 to form the second channel 104. It also leaves a polished surface finish on both the second channel dielectric 101 and the second channel 104.

[0031] After chemical-mechanical polishing, the surface 228 of the second channel 104 is subjected to a low energy nitrogen implantation which passivates the conductive material. For copper, this forms the copper nitride protective layer over the second channel.

[0032] While the best mode utilizes copper as the conductive material, it should be understood that the present invention is applicable to other conductive materials such as aluminum, silver, gold, alloys thereof, compounds thereof, and combinations thereof. Similarly, the barrier layer can be of tantalum, titanium, tungsten, alloys thereof, compounds thereof (especially nitrides), and combinations thereof.

[0033] The preferred embodiment of the present invention is directed to using a dual damascene technique. However, it also will be recognized that other techniques of forming interconnect, such as the single damascene technique, or other traditional techniques of forming low resistance contacts or plugs which involve filling an opening with conductive materials such as tungsten or aluminum may be used to practice the present invention.

[0034] While the invention has been described in conjunction with a specific best mode, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations which fall within the spirit and scope of the included claims. All matters set forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.