BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to semiconductor devices and manufacturing and, more particularly, to methods and structures which prevent degradation in semiconductor device wiring.
2. Background Description
Degradation can occur in metal lines in contact with insulator materials containing fluorine. This degradation is a serious concern because it represents a potential failure mechanism for an integrated circuit, IC. The degradation problem is costly to the industry by virtue of the process monitoring, inspections, and equipment maintenance requirements that it entails.
The performance of advanced semiconductor devices is becoming increasingly limited by the delays in the back-end-of-line (BEOL) interconnections. These delays are dependent on both the resistance and capacitance of the wiring structures, which are partially determined by the material properties (resistivity and dielectric constant) of the conductors and insulators used.
Copper is being used as the conductor for several emerging technology generations to address at least the issue of resistivity. Copper metallurgy formation is possible using a dual damascene integration scheme. In an ongoing effort to lower overall capacitance wherever possible, several low dielectric constant insulators are being investigated for reducing BEOL capacitance. One leading candidate is fluorosilicate glass (FSG) which can reduce the insulator dielectric constant while maintaining many advantages of inorganic, plasma-enhanced CVD films.
Dual damascene wiring consists of wiring trenches and vertical vias between the trenches, which are filled with copper. Typically the copper in the trenches and vias is cladded with a thin layer of refractory metal or metal nitrides containing Ta, Ti, or W. There are a number of specific challenges in using FSG insulators with dual-damascene copper integration. Chiefly, the compatibility of commonly used metallization schemes (liners, diffusion barriers, seed layers, etc) with free fluorine species is of concern. Additionally, the reliability of damascene interconnects is generally thought to be sensitive to the stresses imposed by the insulators and other layers necessary when forming conducting lines and vias. It is recognized that fluorine stability of FSG materials can be increased by careful control of deposition variables. For some FSG films which are otherwise suitable for integration with copper dual damascene interconnections, this stability is inversely related to the mechanical stresses in the FSG insulator films. This relationship, along with the contamination susceptibility of copper interconnections to fluorine species, poses particular difficulties for this integration.
For example, undoped PECVD silane oxide has a relative dielectric constant of 4.3. As SiF4
doping is added to the plasma, the resulting fluorine incorporation into the film decreases the dielectric constant. The fluorine content in the film is typically quantified by FTIR measurement of the Si—F:SiO bond ratio. Film stress correlates with the fluorine content. Table 1 exemplifies the relationship between fluorine content, dielectric constant and stress of some FSG films. Based on reliability data collected on standard homogeneous FSG films (i.e. single layer) integrated into dual damascene copper wiring, we have determined that films with Si—F:Si—O bond ratios above 1.2% can attack the interface between the via and the underlying copper wire resulting in increased via resistance and via opens. Although decreasing the Si—F:Si—O ratio to less than 0.6% eliminates the via interface problem, the high stress of the low fluorine-content FSG film causes significant manufacturing and reliability problems with the resulting structures.
| || |
| || |
| ||Si—F:SiO ||k ||stress |
| || |
| ||0 ||4.1 ||−1.0E9 dynes/cm2 |
| ||<0.5% ||4.1 ||−1.8E9 |
| ||0.6% ||3.8 ||−1.2E9 |
| ||1.2% ||3.8 ||−1.1E9 |
| ||1.9% ||3.7 ||−1.0E9 |
| ||2.2% ||3.6 ||−0.9E9 |
| ||2.5% ||3.6 ||−0.9E9 |
| || |
- SUMMARY OF THE INVENTION
Table 1: Dielectric constant and film stress as a function of fluorine content (Si—F:Si—O bond ratio) for FSG films.
It is therefore an object of the present invention to provide degradation resistance for metals in contact with fluorine-containing insulator materials.
It is another objective of this invention to minimize fluorine poisoning of metals used in IC metallization schemes to reduce undesired via resistance growth and to enhance the contact between a metallization layer and the metal in a via plug.
These and other objectives are achieved in the present invention by providing a metallization insulating structure, comprising:
a substantially fluorine free insulating layer formed on the substrate, having a height, hi;
BRIEF DESCRIPTION OF THE DRAWINGS
a fluorine containing insulating layer formed on the substantially fluorine free insulating layer, having a height, hf.
FIG. 1 is a cross sectional view of a typical line/via configuration
FIG. 2 is a cross sectional view of an embodiment of the structure of the instant invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 is a cross sectional view of an embodiment of the structure of the instant invention.
For purposes of the present invention, the terminology “degradation”, can mean two things. First as used in connection with metal contact and metal-based conductors of electricity, degradation encompasses “corrosion” or “poisoning” of a metal. “Corrosion” of a metal line or metal contact by exposure to fluorine means formation of a metal fluoride compound from fluorine and the metal via chemical reaction. “Poisoning” of a metal by exposure to fluorine means physical infiltration of the metal by fluorine as a contaminant in an amount adequate to increase the contact resistance of the contaminated metal interfaces. Second, degradation is the negative effect on the reliability or electrical properties of the interconnection, due either to the chemical or physical influence of a species.
While not desiring to be bound to any particular theory, it is thought that fluorine-containing insulating materials tend to release fluorine constituents during patterning of metal lines and metal conductors, particularly in the form of fluorine (F) or fluorine gas (F2), which initiates and/or promotes the corrosion and/or poisoning of metals, such as aluminum, copper, tantalum, or titanium, that become exposed to and infiltrated by the released fluorine. This phenomenon has been observed to occur whether the fluorine is an intentional component of the insulator material or even an inadvertent contaminant thereof. For instance, fluorine has been found by the present investigators to be a contaminant in commonly-used TEOS (i.e., tetraethylorthosilicate) based insulator films which are commonly-used as insulating films between metal conductor lines. The presence of fluorine as a contaminant in the insulator layer nonetheless poses a potential degradation threat, once released, to adjacent metal conductor lines.
It has been observed by the investigators of this invention that the rate of corrosion experienced in metal exposed to fluorine is positively related to the concentration of fluorine in the adjoining insulator film; that is, a lower-concentration of fluorine in the insulator film causes less metal corrosion than the case where higher concentrations of fluorine are present in the insulator film.
In general, the thickness of the fluorine-free barrier layer will partially depend on the barrier material and its particular morphology. The thickness of the fluorine-free barrier layer will also be based on the depth of the conducting line that will be formed later. In an embodiment of this invention, the fluorine-free barrier layer is especially useful in a situation where insulator films are intentionally doped with fluorine for the purpose of reducing the dielectric constant of the insulator films in order to reduce capacitive coupling between adjacent metal lines.
This invention relates to a structure with a two component dual damascene dielectric for copper wiring consisting of low stress SiO2 (USG), and low stress SiOxFy (FSG). In this structure, the lower and middle portions of the vias are surrounded by USG; the upper portion of the vias are surrounded by either FSG or USG; and the wiring trenches are surrounded by an FSG insulator. Note the interface between the USG and FSG insulators need not correspond with the intersection of the vias and lines, but could be tailored for optimal performance, manufacturability and reliability. Typically the overall height of the FSG insulator would be greater than the height of the interconnection lines, while still maintaining a fraction of the via height surrounded by USG. Finally note that a layer of silicon nitride or other etch stops could be included between the USG and FSG dielectrics or at the bottom of the wire trench, to allow for a selective trench RIE process and more controlled trench depth.
Referring to the figures generally and FIG. 1 specifically, there is shown a representation of a typical line/via structure. The line portion, 1, has a height represented by hl, and the via portion has a height represented by hv. The overall height of the structure, ht equals hv+hl. When forming the structure of the instant invention, as shown in FIG. 2, a layer of a first undoped insulator, 10, is deposited according to any means known in the art. The height of the undoped layer should not be equal to ht. A substantial portion of the beneficial effects of the structure as outlined in this invention are not achieved when the height of the undoped layer is significantly greater than height of the via, hv. It is preferable if the height of the undoped material is significantly less than the height of the via, hv. A second insulating layer of a doped insulating material, 15, is then deposited. The height of the doped layer should be equal to ht-hv. One of the objectives of the instant invention is to balance mechanical stress needs with increased interconnect capacitance. The balance of those two should be kept in mind when choosing the thicknesses of the doped and undoped materials. Also in a preferred embodiment, a layer of capping layer, 20, would be deposited prior to the deposition of the undoped first insulating layer. The maximum via height, hv should then be greater than the height of the undoped insulating material plus the capping layer material.
The undoped insulating material can be selected from any material known in the art. Preferably, the undoped insulating material would be any silicon dioxide based film which is compatible with back end of the line (BEOL) processing and does not contain any significant fluorine content, whether intention or unintentional. More preferably, the undoped insulating material would be undoped silica glass (USG). Most preferably, the undoped material would be plasma enhanced chemical vapor deposited (PECVD) silica glass from silane or tetraethylorthosilicate (TEOS) precursors. The doped insulating material can be selected from any material where the use of the material causes capacitance/stress tradeoffs. In a preferred final structure should have a mean compressive stress (as-deposited) of between 0.8E9 and 1.4E9 dynes/cm2. Preferably, the doping would be fluorine. The fluorinated insulators could be any insulator with sufficient fluorine content to have a risk of degradation when integrated with metallization susceptible to fluorine poisoning. The fluorinated insulating material could be any of the following, including but not limited to, fluorinated silicon dioxide, fluorinated amorphous or diamondlike carbon or fluorinated organic polymers. Preferably, the doped insulating material would be fluorinated silica glass (FSG). The capping material could be any material with suitable etch selectivity relative to the chosen undoped insulator, appropriate copper diffusion barrier properties and other properties compatible with BEOL processes. An example of preferred embodiment capping layer materials include, but are not limited to silicon nitride, silicon carbide or hydrogenated silicon carbide. Preferably, the capping layer would comprise silicon nitride SixNy.
- EXAMPLE I
Each of the three layers could be deposited by any means known in the art. Examples of deposition methods for depositing the undoped layer include any PECVD or HDP-CVD processed silica film, for example from SiH4 or tetraethylorthosilicate (TEOS) precursors, provided that the average stress of the bilayer stack is as described above. The FSG film would similarly preferably be either PECVD or HDP-CVD, using either SiH4 or TEOS precursors and using any of a numerous fluorine sources, including SiF4 or C2F6. The dielectric constant of the FSG film can range from about 3.5 to about 3.9. An example of the method leading to the final structure of the instant invention is given in Example I.
In the instant invention we are presupposing that hv=0.6 μ and hl=0.4 μ. An underlying wiring level is capped with 700A of silicon nitride, A 0.4 um layer of undoped silica glass (USG) is then deposited. A layer of doped silica glass, FSG, 0.53 μ is then deposited. The FSG film which is used is a dual frequency PECVD film, deposited from SiH4, N2O, inert carrier gases and SiF4 as the fluorine doping gas. The deposition is at 380°-400° C., with 2000W of total RF power. The dielectric constant of the film is about 3.75, the refractive index is about 1.445 and the as-deposited stress of the film is about 1.5E9 dynes/cm2 in the compressive direction. The USG film is a single frequency PECVD film, deposited from SiH4, N2O and inert carrier gases. The deposition is at 380°-400° C., with 1100W of total power. The dielectric constant is about 4.1, the refractive index is about 1.46 and the stress is about 0.8E9 in the compressive direction.
After forming the two, or three layers metallization can occur. The metallization can be formed, or patterned, using any process known in the art but preferably, a dual damascene process would follow. It should be noted that the instant invention is independent of the metallization deposition process. When a dual damascene formation process is used either a line first or a via first approach is possible. A representation of the insulation barrier layers with the metallization is shown in FIG. 3, where the capping layer, 20, is silicon nitride, the undoped layer, 10, is USG, the doped layer, 15, is FSG and the metallization, 30, is copper.
While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.