Patent Grant
7223685
The present invention relates generally to semiconductor wafer processing and in particular, but not exclusively, to electrochemical removal of barrier layers from low-k dielectric layers in semiconductor wafers.
Semiconductor devices usually include a large number of features or components—such as transistors, switches and conductive lines—built on an underlying substrate or wafer. The components are usually built by successively depositing layers of different materials on the substrate and then etching and/or selectively removing all or part of the deposited layers. The deposited layers are of different materials depending on the component, but can include metals, metal alloys, pure semiconductors, doped semiconductors, and dielectrics.
Certain semiconductor devices include a variety of conducting paths or interconnects between components of the device. These interconnects are often built by etching a feature such as a trench into a dielectric layer, and then depositing an adhesion layer, a barrier layer and, finally a conductive layer onto the dielectric layer. To complete the interconnect, the conductive layer, barrier layer and adhesion layer must be removed from the regions of the dielectric layer surrounding the feature (also known as the “field”), leaving the trench filled with a conductive layer, usually metal, separated from the dielectric layer by the barrier layer and the adhesion layer.
The method of choice for removing conductive and barrier layers from a semiconductor wafer has been chemical mechanical polishing (CMP). In CMP, a mildly abrasive slurry is poured onto a polishing pad, and the wafer surface is then pressed onto the slurry with a force calculated to exert a certain pressure on the surface of the wafer. The polishing pad and the surface of the wafer move against each other causing the abrasive slurry to grind away the conductive or barrier layers on the surface of the wafer. Despite its prevalence, however, CMP has some important disadvantages. CMP is inherently expensive because it uses substantial amounts of consumables that cannot be re-used, such as polishing pads and abrasive slurry. Because CMP involves polishing the surface of a wafer by the exertion of a mechanical shear stress on the surface of the wafer, CMP can easily damage structures on the wafer. When metal and barrier layers are used on a wafer to form a structure in a dielectric material with a low dielectric constant (also known as a low-k dielectric), CMP has the potential for large amounts of damage. Low-k dielectrics have correspondingly low material properties, such as Young's modulus, hardness, toughness, etc, meaning that mechanical stresses can be particularly damaging. Since damage to even a small number of structures on a wafer can render the entire wafer useless, use of CMP, particularly with low-k dielectrics, can substantially lower the yield and raise the expense.
There have been attempts to use a hybrid method that combines CMP with electrochemical removal of layers from a wafer, but these attempts have not had satisfactory results. Because the hybrid method continues to rely on mechanical forces, it carries with it all the disadvantages of CMP methods. When metal and barrier layers are used on a wafer to form a structure in a dielectric material, electrochemical removal has yielded poor results. Traditional barrier materials are very resistant, meaning that they require very high applied potentials for removal. Moreover, traditional barrier materials have been difficult to remove without also removing the metal layer; in other words, existing electrochemical approaches are not selective enough to the barrier materials used.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Embodiments of an apparatus and method for electrochemical removal of layers in semiconductor wafers are described herein. In the following description, numerous specific details are described to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
The feature 104 is created in the ILD layer 102 using ordinary masking and etching processes known in the art. The ILD layer 102 can comprise any type of dielectric, although a dielectric having a low dielectric constant k (i.e., a low-k dielectric) is preferred to reduce ill effects from cross-capacitance that occurs, for example, between neighboring interconnects.
After the feature 104 is created in the ILD layer, the under-layer 110 is deposited on the surface of the ILD, such that it coats the bottom 106 and sidewalls 108 of the feature 104, as well as the field surrounding the feature on the wafer. The under-layer 110 promotes adhesion and serves as a conductive layer for electro-dissolution of the barrier layer 112. In various embodiments, the under-layer comprises materials such as titanium (Ti), titanium nitride (TiN), tungsten (W), tungsten nitride (WN) or tantalum nitride (TaN), although other materials are possible in other embodiments. The under-layer can be applied to the ILD layer using chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD). In a preferred embodiment, the under-layer is deposited using PVD to provide a thicker conductive layer on the ILD surface while providing a thinner layer on the bottom 106 and sidewalls 108 of the feature 104.
Following the deposition of the under-layer 110, a barrier layer 112 is deposited on the under-layer. The barrier layer should preferably be conductive enough to allow electro-polishing of the conductive layer 114, and so that it lowers the electrical resistance and requires less applied potential difference. In various embodiments the barrier layer 112 can comprise ruthenium (Ru), rhodium (Rh), tantalum (Ta), iridium (Ir), osmium (Os), or alloys thereof containing nitrogen (N), silicon (Si) or carbon (C). As with the under-layer 110, the barrier layer can be deposited using CVD, PVD or ALD.
Finally, following deposition of the barrier layer 112 an electrically conductive layer 114 is formed on the barrier layer 112 using conventional processes such as CVD, PVD or ALD. The deposition of the conductive layer is customarily followed by electroplating to fill the feature 104, or at least those portions of the feature not already filled with the under-layer and barrier layer, with conductive material as shown. In the illustrated embodiment the conductive material used for the conductive layer is preferably copper (Cu), although other conductive materials may be suitable in other embodiments.
Although the illustrated embodiment shows the vessel 202 as a beaker, in other embodiments the vessel 202 can be any kind of vessel or container capable of holding a fluid; the exact size, shape and construction of the vessel will be determined by operational requirements, such as the number of wafers to be simultaneously processed and the sizes and shapes of the individual wafers.
In one embodiment of the electrolysis cell 200, the electrolyte 204 includes a base and has a pH equal to or greater than 10. Suitable bases for the electrolyte include solutions of potassium hydroxide (KOH), sodium hydroxide (NaOH), ammonium hydroxide (NH4OH) or tetra-methyl ammonium hydroxide (TMAH). In alternative embodiments, certain additives can be included in the electrolyte to accomplish specific purposes. Oxidizers such as hydrogen peroxide (H2O2) can be added to the electrolyte to increase the rate of electrochemical dissolution. Corrosion inhibitors such as benzotriazole can be added to protect the copper that will be left behind in the feature from corrosion due to electrolysis. Surfactants such as TRITON-X®, manufactured by the Dow Chemical Company, can be added to increase the selectivity of the electrolysis and to protect the conductive layer. Buffers such as potassium carbonate (K2CO3) can be added to control the pH of the electrolyte. Finally, complexors such as potassium citrate can be added to enhance dissolution in the electrolyte. A limited number of embodiments of the additives are illustrated in Table 1; additional or different additives are, of course, possible within the scope of the invention.
The anode comprises the wafer 100, which in the state shown in
The electrode 206 forms the cathode or counter electrode, whilst the wafer 100 forms the anode, which in this case is the working electrode. In addition to the wafer (anode) and the cathode 206, a reference electrode 208, which in one embodiment is a saturated calomel electrode, is also at least partially immersed in the electrolyte 204.
All three electrodes—the wafer (anode) 100, the cathode 206 and the reference electrode 208—are electrically connected to a potential source 210, which applies an electrical potential difference to the electrodes to cause electrolysis to occur. The potential source can be any kind of source capable of applying a voltage having a specific value with respect to a reference electrode, such as the SCE. In one embodiment, the voltage can have a value greater than or equal to 0.5V relative to the SCE. The source may be as simple as a common battery, although in most applications a potential source whose potential difference is steady and accurately controlled is preferred.
In operation of the electrolysis cell 200, a potential difference is applied between the wafer 100 and the cathode 206, causing the electrolytic removal of the barrier layer 112 to begin. As the electrolysis proceeds, the barrier layer 112 of the wafer (see
The above descriptions of embodiments of the invention and the description in the Abstract below are not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. These modifications can be made to the invention in light of the above detailed description.
The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be interpreted in accordance with established doctrines of claim interpretation.
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