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Spectroscopic Ellipsometry for Copper and Low κ Process Development

by Arun R. Srivatsa and Carlos L. Ygartua, KLA-Tencor Corporation

Spectroscopic Ellipsometry (SE) is a fast, non-destructive technique used for the routine production monitoring of single layered and multilayered thin film structures. In addition to thickness, the refractive index (RI) can be monitored at one or more appropriate wavelengths for a single layer or multiple layers in a multi-layered structure for greater process control. The dispersion (variation of RI with wavelength) can be correlated to the microstructure and composition of the film. Processes can be characterized by monitoring the change in RI with process conditions at appropriate wavelengths chosen for maximum sensitivity to process variations. In this article, we review some of the recent applications of SE for copper (Cu) and low κ process development.

Considerable work is underway in the development and integration of Cu and low κ processes(1-3). Most of the current and immediate Cu-based technologies utilize oxide or oxide-like materials with appropriate barrier materials to form the interlayer dielectric (ILD) structure. Simultaneously, much work is ongoing in the development of low κ materials and resolution of process issues, since the benefits of Cu technology are better realized by integration with low κ dielectrics. Materials being studied for low κ ILD structures can be broadly classified into three categories: a) spin-on polymers, e.g., SiLK™, FLARE™ and BCB; b) chemical vapor deposited (CVD) films, e.g., Black Diamond, Coral™, SiOF, BLOK™ and c) highly porous films for ultra low κ applications, e.g., Nanoglass.

The porous films are candidates for ultra low κ applications, potentially several years from now. In the near term, for materials with dielectric constants around 2.7, the choice is between the spin-on polymers and the CVD deposited films.

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In routine production, the ability to monitor the stability of a process is critical. From a metrology standpoint, therefore, the capability to measure and monitor the different kinds of low κ materials and multilayered ILD structures on silicon or Cu substrates is essential. In this article, we demonstrate the capability of SE for materials characterization and production monitoring using several examples. As there are many reviews on SE, this article does not go into any details on the technique(4-5).

Monitoring the cure of SiLK

The dispersion characteristics of spin-on polymers like SiLK change with annealing (curing). A thin film of SiLK is formed by spin-coat application of an oligomeric solution. This is subsequently cross-linked by a cure process. Since the mechanical properties of SiLK depend on the degree of cure, it is essential to monitor the curing process using a rapid non-destructive technique.

To determine the usefulness of SE for monitoring the cure process, a time-temperature annealed wafer set provided by researchers from Dow Chemical was analyzed. The wafer thicknesses, annealing conditions and measurement results are summarized in Table 1. All measurements were carried out on a KLA-Tencor ASET-F5 thin film measurement system. A simple three-term BEMA model was used for the analysis.

The dispersion characteristics of the SiLK were also

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seen that there are two peaks, both of which decrease with increasing cure. A consistent, gradual change in index is observed for the more extensive time-temperature anneal data summarized in Table 1. The effect of the anneal on the index at 314 nm is also clearly seen in the plot in Figure 1b, which includes a subset of the data from the Table, showing the variation of the index with annealing time at a constant temperature. From this, it is seen that 314 nm is best suited for monitoring the cure of SiLK. The total magnitude of the change in RI is around 0.08 at 314 nm. While this is large enough to monitor the cure process in production systems, more statistical experiments need to be carried out to determine the repeatability of the cure process and the degree of resolution of the cure.

Table 1. Var iation of RI of SiLK wit h a nnealing condition s.

1a)

Porous films – void fraction and correlation to RI

1b)

F i g u re 1a) . Variation in di spersion characteri sti cs of SiLK as a function of annea ling b) C han ge in RI at 31 4 nm with an nealing time at constant temperature (400°C)

independently validated by analysis of spectra from a research grade spectrometer (GESP-5) using variable angle spectroscopic ellipsometry.

The thickness of the SiLK films for all wafers was around 7300 Å. Prior to the annealing step, all samples, including the control, were exposed to a soft bake at about 320º C. The optical dispersion of the SiLK was analyzed as a function of cure. It was found that the effect of the annealing was most pronounced on the optical properties of the SiLK in the region around 300 nm, especially at 314 nm. The variation in the dispersion (n – real component of refractive index) profile in this region for three different anneal conditions is plotted in Figure 1a. It can be

By mixing material with void and creating porous films, ultra low dielectric constants (below 2) can be achieved. The dielectric constant of the films is a function of porosity. The mechanical properties depend on several factors, total porosity, pore size and distribution.

Nanoglass is an example of a porous, SiO2 based film. These materials are typically formed by spin coating. A typical nanoglass spectrum and dispersion plot is shown in figure 2a. Note that the nanoglass has an index close to 1 across the entire spectrum suggesting a highly porous film. A simple, two-term BEMA model using

2a)

2b)

F i g u re 2. a) Tan(psi)- C os(delta) sp ectra of a nanoglass fil m on sil icon. b) Typi cal dis persion of a nanoglass.

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silicon dioxide and void was used for the analysis. The void fraction or porosity can be directly derived from the model.

Analysis of several nanoglass samples revealed a linear relationship between the refractive index of the film (reported at 633 nm) and the void fraction (Figure 3).

Therefore, by measuring the RI of the film, it is possible

Measurement of a mutilayered dielectric film structure

A) Seven Layer Filmstack on Si The following example illustrates the multilayer measurement capability of SE. A seven-layer dielectric film stack was built up from the silicon substrate with the following target thicknesses: (Si/oxide (5500Å) nitride (1000Å) BCB (7000Å) oxide (1000Å) nitride (1000Å) BCB (7000Å) oxide (1000Å) The uniformity of the layers on this 200 mm wafer was evaluated by doing a wafer map with a 10 mm edge exclusion zone with a KLA-Tencor UV-1280SE. To avoid correlations, the thickness of the middle oxide layer was fixed. Thicknesses of the other six layers and the index of the bottom BCB layer, a total of eight parameters, were measured.

F i g u re 3. Variation of refractive index (n a t 633 n m) wit h void fract ion in silica ba sed nan oglass and xerogels. Not e the linear re l a t i o n s h i p bet ween ind ex and voi d fraction.

Wafer contour maps for the top and bottom BCB layers are shown in Figure 4. The thicknesses are in line with the nominal thicknesses, and the ranges are consistent and similar to thickness range observed for a three layer stack with a single BCB layer as reported previously(6).

B) Four Layer Dielectric Filmstack on Cu to compute the void fraction. For nanoglass and xerogels, the pore size and distribution are parameters of interest. While SE is extremely sensitive to small changes, a systematic study needs to be undertaken to determine if these additional parameters can be monitored using this technique.

Vapor deposited, oxide-like and other low κ films

We have analyzed a variety of single layer and/or multilayered filmstacks with low κ films formed by CVD and/or other techniques. A partial list includes, SiLK, BCB, FLARE, FSG, HOSP, TY7, HSQ, Black Diamond, BLOK and Nanoglass. Many of the CVD deposited films like Black Diamond are oxide-like in nature and quite easy to characterize. The BLOK (barrier low κ) films appear to have both C and O. The composition of these films is tuned depending on the application and, consequently, there is a wide range of RI values for the BLOK films.

Multilayer measurement capability on Cu is illustrated in Figure 5. A four layer filmstack with oxide/nitride/ oxide/nitride on Cu was measured using the ASET-F5.

The Cu layer in this case is thick enough to absorb light and serve as an “effective” substrate. Contour maps of all the layers were simultaneously obtained, 1kÅ oxide 7kÅ low- κ 1kÅ nitride 1kÅ oxide 7kÅ low- κ 1kÅ nitride 5.5kÅ oxide Silicon F i g u re 4. Schema tic of seven layer filmstack with two layers of BCB and contour maps of th e “top” and bottom BCB layer s.

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3kÅ oxide

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ments without moving the wafer. The 1 sigma standard deviation for each of the layers was less than 0.15 Å.

500 Å nitride 7kÅ oxide 1kÅ nitride COPPER Tantalum Oxide

Monitoring the oxidation of copper

SE offers an easy way to monitor the oxidation of copper.

The dispersion plots for copper, cuprous oxide and cupric oxide are shown in Figure 6. It is seen that there are substantial differences between pure copper and the different oxides. Therefore, by monitoring the dispersion of the film, it will be possible to identify the presence of either a pure single phase or mixtures of phases.

Silicon F i g u re 5. Dielect ric filmstack on copper with targeted thicknes ses indi cated. Wafer contour maps for t he in dividual layers are s hown on the righ t. The short ter m repeatability (si ngle site – 30 repeat measurements) f or a simulta neous measurement of all 4 la yers was less than 0.15 Å, 1 sig ma for each la yer.

with an edge exclusion zone of 6 mm. The contour map provides a means for mapping the deposition reactor “signature” profile. The contour maps for both the oxide layers, which are formed on top of nitride layers, have similar characteristics. There are some differences in the two nitride contour maps, possibly due to the differences in deposition of nitride on silicon and nitride on oxide. Thicknesses of all the layers were determined correctly (verified subsequently by crosssectional analysis), despite the fact that the nominal thicknesses were not provided prior to the measurement.

The robustness of the measurement was also verified using a standard “precision” test – 30 repeat measure-

Interfacial reactions between copper and silicon dioxide

Copper tends to react in the presence of oxygen. This is one of the reasons for a thin nitride barrier between silicon dioxide or oxide-like low κ materials and copper.

An example of the sensitivity of SE to the variation of the interfacial reacted film is shown in Figure 7. In this example, silicon dioxide film about 7000 Å thick, deposited directly on a copper film was analyzed. An interfacial, reacted layer was found with a thickness that varied from about 100 – 300 Å across the wafer.

In addition, the dispersion of the reacted layer varied substantially across the wafer indicating variation in composition of this layer. The two spectra reproduced in Figure 7, from different locations of the wafer, show the clear differences between the films. Note the differences in the spectral features in the wavelength range from about 350 to 650 nm between the two spectra. It is interesting to note that significant differences were observed even over very small areas within a zone of 1 mm diameter.

F i g u re 6. Disp ersion ch aracteri stics of copper, cuprous and cupr ic oxi de.

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for copper and low κ process development and process monitoring.

Acklowledgements

We thank our customers and collaborators for providing samples for analysis.

References 1 . C.S. Chang, K.A. Monnig, M. Melliar-Smith, Proceedings of the IEEE 1998 International Interc o n n e c t Technology Conference, 3 (1998).

F i g u re 7. Tan(psi)-Cos (delta) spectra from diff e ren t a rea s of a sil icon di oxide fi lm on copper. Diff e rences are pr imarily due to the d iff e renc es in the interfacial layer.

2 . C.H. Ting and T.E. Seidel, Mater. Res. Soc. Symp.

P roc., V381, 3 (1995).

3 . S . P. Murarka, Mater. Sci. Eng. R, V19, 87 (1997).

4 . K. Vedam, Thin Solid Films, 313-314, 1 (1998).

Summary

SE is a powerful technique that can be used both for process characterization and routine process monitoring. Using the wafer mapping capability in production systems, the “signatures” of the deposition reactors can be determined and monitored. The examples discussed in this article illustrate the usefulness of this technique

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5 . R.M.A. Azzam and N.M. Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam (1997).

6 . A.R. Srivatsa and C. Ygartua, in “Optical Metrology” Ed., Ghanim Al-Jumaily, SPIE Optical Engineering P ress, p 61 (1999). circle RS#002

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