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Application of Spectroscopic Ellipsometry-based Scatterometry for Ultrathin Spacer Structure Ryan Chia-Jen Chen, Fang-Cheng Chen, Baw-Ching Perng, Yuan-Hung Chiu, and Hun-Jan Tao, Taiwan Semiconductor Manufacture Co. Ltd. Ying-Ying Luo, KLA-Tencor Corporation

Many applications for spectroscopic ellipsometry-based (SE) scatterometry have been developed for critical dimension (CD) control of the resist patterning and poly gate processes. Scatterometry’s many advantages include good precision, short cycle times, and multiple information outputs. The spacer structure has now emerged as a promising new application for SE-based scatterometry. In this work, we use SE-based scatterometry to demonstrate a two-dimensional profile of the ultrathin spacer with post-etched structure, as well as CD measurement of the spacer. Theory and measurement results taken of dense and isolated structures will be briefly discussed. Transmission electron microscope (TEM) cross-sections and the spectra fitting by scatterometry are also collected at the same location and compared. The data shows high correlation between the two. Finally, an example of minispacer fault detection methodology and repeatability test using scatterometry is also presented to show the technology’s capability for volume production.

Inroduction

The minispacer structure has been actively studied as a means to maximize transistor performance and suppress the degradation of short channel effect (SCE)1 for sub-micron technology. Normally, the minispacer has a width ranging from 100Å to 250Å.

However, its width could also be as thin as 50Å (or even lower) when the channel length dimensions continue to shrink in order to meet the requirements for device speed and to reduce SCE. The ability to measure the width of the minispacer with good precision control is paramount to device performance, because it will influence optimization of the implant condition. However, there is currently no inline metrology tool that is capable of measuring the minispacer width because it is too thin to be measured.

Traditionally, a TEM is used to monitor the minispacer profile. However, TEMs have several drawbacks, such as their tendency to

be destructive, offer low throughput, and involve long sample preparation times. This technology is also not practical for volume production, when statistical sampling is required. Recently, there have been many studies for SE-based1 metrology used in lithography2, polysilicon gate3, and shallow trench isolation (STI)4, as well as on its precision control and correlation to scanning electron microscopy (SEM). In this paper, the measurement capability of SE-based metrology on minispacers down to 50Å is evaluated.

Experiments and results

Experimental In this study, all scatterometry measurements were performed with KLA-Tencor’s SpectraCD, an SE-based scatterometry system. The spectroscopic ellipsometry technique is widely used for thin film metrology.

SpectraCD utilizes a grating structure comprised of line/space features of uniform period to represent the device feature. The box size of the grating structure is designed as 50x50 µm in order to cover the spot size.

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with line and space ratios of 100/400 and 100/150 respectively, are chosen for this work. SpectraCD measurements include off-line library generation and tool system measurement. Library generation involves material dispersion characteristics, nominal film thickness, and grating information such as height, CD, and profile. As is typical of spectroscopic ellipsometry measurement, the wavelength range of incident light from 250 nm to 750 nm is applied for spectra collection and calculation.

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All SE measurement results are compared to the respective measurements obtained from TEM for reference and correlation as well as profile information validation.

Figure 2. Nominal film stack with incident light interaction with Films

Measurement models and simulation A schematic representation of the film stack after minispacer etch is shown in Figure 1. The structure includes polysilicon, gate oxide, and minispacer formed on both sides.

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Figure 3. Effective dielectric model based on EMA assumption.

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Figure 1. Nominal minispacer film stack.

The layers of interest comprise Film 1 and the underlying Film 2 (see Figure 2). Film 1 and Film 2 are dielectric films, such as nitride, oxide, or oxynitride-like film. As the total thickness is less than about 50 nm, the model can be further modified as one effective dielectric layer and be calculated based on the Bruggeman effective medium approximation (EMA) theory5, where it is assumed as a random mixture of constituents. Therefore, the film model can be modified as shown in Figure 3.

The Bruggeman EMA has shown to be quite useful for calculating the dielectric functions for films of mixed composition with thickness of 5 nm to 50 nm. The final measurement and simulation model is shown in Figure 4.

The indicator of spectra fitting quality used to evaluate matching of simulation result to measurement result is the quantity of chi-squared.

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The simulation result for an iso-grating structure is shown in Figure 4. Two experimental spectra, 伪(位) and 尾(位), are obtained from the SE measurement and represent the polarization of polarized reflected light.

They are the characteristics of film stack, thickness of layers, and refractive indices, and extinction coefficient of layers and substrate. As the thickness of the interest layer increases, the spectrum curve behaves differently at the regions of ultraviolet (UV) and visible light area.

The UV wavelength region is important for spectroscopic ellipsometry to accurately measure a dielectric layer such as nitride film because it is more sensitive to index of reflection (n) and absorption coefficient (k).

For the two-dimensional profile measurement, as seen in Figure 4, we observed greater changes of spectrum in the UV wavelength region as this combination film thickness changes. This observation correlates with the simulation result of the real film stack. Similar behavior occurred on the dense grating structure, although

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Therefore, there is a reasonable process window for setting up the EMA model. In all cases, the normalized goodness-of-fit (NGOF) is above 0.98.

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Figure 4. SE simulation result at different spacer width.

different spectra patterns were observed. When the effective medium approximation is applied to a twodimensional profile, we also need to take into account that poly film can interfere in those two effective dielectric layers with the incident light, and cause the approximation to deviate. Therefore, the combination ratio of these two dielectric layers in the EMA model may not be the same as the physical result, and needs to be modified. Figure 5 includes chi-squared results for the assumption of the different ratio combination EMA model. The lowest chi-squared data is obtained at model E for best fitting. There is less than a 10Å change from model B to model E with the reasonable chi-squared values. This result indicates that the measurement is insensitive to minor changes of the model.

Figure 6a shows a TEM cross-section of a test structure with a 40Å to 70Å ultrathin minispacer on both sides of the poly gate. Figure 6b is a top-down SEM image of the same structure. We can barely identify the interface between the polysilicon layer and the minispacer through the SEM picture. It is beyond the ability of current generation CD SEM tools to distinguish the interface between the poly gate and the minispacer, and obtain a robust measurement of the minispacer width. In order to ascertain the precision capability of SE-based scatterometry, four minispacer wafers were prepared with different spacer widths of 50Å, 110Å, 220Å, and 450Å. The thickness ratio of Film 1 to Film 2 is different for each spacer width. Not every die on the wafer has a TEM cross-section result because it takes a long time for a TEM sample to be prepared. However, selected die have SE data and the corresponding TEM cross-section result, allowing us to plot the relation of SE and TEM data. The EMA model is also adjusted according to the lowest chi-squared fitting and film deposition thickness. Strong correlation, with regression of 0.99, is observed in Figure 7 for the spacer width range of 40Å to 450Å. In other words, this EMA model setup is valid for real two-dimensional simulation.

Moreover, this SE technique can be applied not only to the ultrathin spacer application for the purpose of reducing the short channel effect, but also to the regular main spacer.

As minispacer widths go below 100Å, a major concern would be the sensitivity and limitation of the SE technique to detect the spacer width. We designed an experiment to obtain an ultrathin offset spacer of 50Å width, and altered the spacer width by three different

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EMA Film Model Figure 5. The impact of different EMA combination model to chisquared fittings.

Figure 6a. TEM cross-section of a test structure with a 40Å to 60Å ultrathin minispacer on both sides of the poly gate. Figure 6b. Top-down SEM image of the same test structure.

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Figure 9. Comparison of SE result and TEM cross-section across wafer.

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split conditions. In Figure 8, under three different split conditions and by increasing the percentage on etch time, the spacer width decreases from 40Å to 36Å and 25Å. Both SE and TEM data show similar process trends, with an average of 10Å offset between the two.

Scatterometry is able to detect subtle 5Å to 10Å changes of this ultrathin minispacer. There are two possible reasons for the 10Å offset between SE and TEM: 1) EMA model approximation; 2) TEM measurement error. The measurement error could be the main reason, as we experienced difficulty in accurately distinguishing the true interface between this ultrathin layer and the TEM cap layer.

After proving its ability on the ultrathin spacer measurement, we applied the scatterometry technique to the wafer uniformity study, since the understanding of uniformity is critical for any process to be considered for volume production or process improvement.

Because the minispacer has very thin spacer width, the

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Parameter Wafer Location Left Top Center Bottom Right

NGOF Spacer Width Poly MCD Chi2 Rectangle Ellipse Rectangle Ellipse Rectangle Ellipse Rectangle Ellipse 175 339 0.991 0.991 12.9 14.6 55.59 55.97 375 0.991 0.990 14.0 15.8 56.41 56.68 121 53.52 54.14 27 430 0.989 0.987 14.0 17.2 287 0.989 0.990 12.9 14.8 55.58 56.15 338 553 0.987 0.983 14.5 15.0 57.17 57.98 364

Table 1. Spacer width result comparison between rectangular and elliptical spacer profile models.

Spacer profile modeling is not included in the main discussion because very thin spacers would not make too much difference on the spacer profile in our study.

Table 1 shows the comparison of the measured results of the spacer width based on rectangular and elliptical simulation models. There is a 1 nm to 2 nm offset between these two models, and a similar trend was observed from site to site. From the TEM cross-section, it is not possible to make a clear judgement on the spacer profile. In both cases, the polysilicon middle CD results are almost identical.

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variation of width from site to site could be as small as 5Å to 10Å. This is beyond the capability of current inline metrology tools. Using the inline scatterometry tool, we examined the wafer uniformity and were able to detect this small difference in process variation. We selected eight different dies across the wafer for SE and TEM comparison (Figure 9)—spanning dies from the extreme left edge of the wafer to the extreme right, as well as top and bottom edge die—marked from P1 to P8. The SE data, simulated with two different models, matches very well with the TEM result. There is about a 5Å difference on average between the SE and TEM results, equal to 2.2 percent of total film thickness.

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Fault detection function Unlike a CD SEM, which provides an image converted from the scanning of the electron beam, the SE mea-

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results, and includes polysilicon middle CD, polysilicon height, sidewall angle, and minispacer width on 5 different individual dies after 10 dynamic tests. The results are excellent and well below the spec limit, with the highest 3 sigma minispacer width of 0.3 nm.

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Table 2. Precision results of 10 dynamic repeatability tests.

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Figure 10: Spacer width and polysilicon middle CD for rectanglular and elliptical models.

surement result is based on spectra fitting through the interaction of optical incident lights and the grating structure. The SE measurement does not provide a real image for visible inspection and fault detection. The SE tool relies on the results of the spectra fitting for fault detection. In this experiment we prepared a non-minispacer wafer to be measured with scatterometry and compared with the nominal wafer result. Wafer slots 1 to 15 have nominal minispacers and wafer slots 16 to 24 have no spacer. Figure 11 shows dramatically different chi-squared values when comparing both types of wafers.

Non-minispacer wafers were observed to have high chi-squared values, indicating poor spectra fitting. On the other hand, nominal wafers had good spectra fitting and consistent 200 to 300 chi-squared value. NGOF values on all wafers were above 0.98. The result tells us that chi-squared is a good indicator for fault detection.

As we set specific chi-squared values for the threshold limit, they can be used for fault detection.

Precision and repeatability Without good precision and repeatability, the measurement data will be meaningless for statistical analysis and publication. We, therefore, examined the repeatability with 10 dynamic tests. Table 2 lists 4 different precision Chi-squared Indicator in Mixed Lot Spacer

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Conclusion

We have successfully demonstrated scatterometry for ultrathin spacer as well as thick spacer measurements.

The Bruggeman EMA model assumption is valid for minispacer measurement. Not only can it simplify the model, but it is also more practical for spacer width measurement. From our EMA simplified model, scatterometry is also capable of identifying the minor changes of minispacer width on either over-etch split cases or variation across wafers. The fault detection is as important as spacer width measurement. Chi-squared value from spectra fitting can provide the functionality of fault detection with certain threshold limits set on the tool as seen in the presented result. We also see that SE metrology provides high precision on the dynamic repeatability test and a high confidence level for volume production.

Acknowledgement

The authors wish to thank Mike Slessor and Peter Huang of KLA-Tencor for useful discussion and help in this effort.

References 1. S. Thompson et al., VLSI Tech. Dig., p. 132-133 (1998).

2. X. Niu and S. Yedur, “Specular Spectroscopic Scatterometry in DUV Lithography”, Proceeding of the SPIE, Vol. 3677, pp. 159-168, Mar. 1999.

3. C.J. Raymond, M. Littau, R. Markle, M. Purdy, “Scatterometry for Shallow Trench Isolation (STI) Process Metrology”, Proceeding of the SPIE - Int. Soc. Opt. Eng. 4344, P716-25, 2001.

4. C.C. Baum et al., “Scatterometry for Post-Etch Polysilicon Gate Metrology”, SPIE Microlithography Conference 1999, Proc. SPIE 3677, p.148, 1999.

5. D. Bruggeman, Ann. Phys. (Leipzig) 24, 636 (1935).

Slot Figure 11. Chi-squared result in mixed lot spacer.

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