A Non-Destructive Metrology Solution for Detailed Measurements of Imprint Templates and Media

Jeffrey Robertsa, Linlin Hua, Torbjörn Erikssonb, Kristian Thulinb, Babak Heidaric

an&k Technology, Inc., 80 Las Colinas Lane, San Jose, California, USA, 95119; bObducat Technologies AB, P.O. Box 580, SE-201 25, Malmö, Sweden

cObducat AB (publ), P.O. Box 580, SE-201 25, Malmö, Sweden

ABSTRACT

This study investigates a non-destructive optical metrology technique, that furnishes measurement solutions for hard drive discrete track recording (DTR) and bit patterned media (BPM) templates and imprints. From the measurement and analysis of polarized reflectance and transmittance, feature height and profile of DTR and BPM templates and imprints, as well as residual layer thickness of imprints, are accurately determined, and uniformity maps of these parameters are produced in a fraction of a minute. Simulations of theoretical polarized reflectance and transmittance, relating to next generation structures, demonstrate that the optical metrology solution has capability for future products. Keywords: optical metrology, RCWA, Forouhi-Bloomer dispersion equations, DTR, BPM, imprint lithography, residual layer thickness, patterned media

1. INTRODUCTION

The drive for higher capacity in storage devices has forced disk drive manufacturers to develop technologies to improve storage density. Media patterning allows the magnetic isolation to be improved, thereby increasing the recording capacity of the disk. The patterning requires a mastering technology to write a template and a printing technology to mass-produce the patterning onto disk substrates. The lithographic demands, however, far outstrip the dimensions obtainable through the widely accepted UV lithography approaches.

Patterning of magnetic media presents complex technology challenges for the disk drive manufacturer. The physical property limitations of the disk itself, i.e. the polycrystalline nature of the magnetic medium, set a limit for the maximum storage density of the written information. At very high densities, above 500GB per square inch, the small dimension of the grains (~10nm) is not sufficient to stabilize the magnetization direction over long time periods due to issues such as thermal stability. A way to circumvent this limitation is to use patterned media, where discrete track recording and bit-patterned media are the most promising technologies. This patterning is done by lithography, although the requirements are not at all similar to those of conventional CMOS lithography. Pattern resolution exceeds the current semiconductor lithography roadmap by more than one generation, and the common stepper technology would not be suited to this application due to stitching errors, i.e. the small misalignments that occur when one field is placed next to another, feature to feature. Patterned magnetic media, in combination with other techniques, is expected to extend the data densities up to 50TB per square inch1.

By using patterned media, data densities can be increased beyond the superparamagnetic limit. Ordinary thin-film media consist of small, single domain magnetic grains that are exchange-decoupled from one another. The magnetic axes of these grains are randomly distributed. For a good signal-to-noise ratio (SNR) in a written bit, each bit must contain a number of grains. To increase storage densities, bit sizes (and therefore grain sizes) must be reduced to much less than 10nm in diameter. However, the grain size cannot be reduced arbitrarily, because as the grain volume decreases, thermal effects become significant. When the magnetic grains are decreased in size, the magnetization directions of the grains can reverse spontaneously. This leads to loss of recorded data.

Photomask Technology 2009, edited by Larry S. Zurbrick, M. Warren Montgomery, Proc. of SPIE Vol. 7488, 74881Z · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.833465

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Manufacturing patterned media presents new challenges and its implementation requires two technologies, a writing technology (electron beam recorder) and a printing technology (nano-imprint lithography). New measurement technologies will be needed to support the development and manufacturing of these technologies. Optical scatterometry is one measurement technology that can play an important role in manufacturing, if proven successful for patterned media. Unlike SEM and AFM, optical metrology can provide fast and non-destructive measurements for feature height, CD and under layer film thickness. This paper focuses on the measurement of templates and NIL produced substrates using a polarized reflectance and transmittance based scatterometer.

2. THE CHALLENGE OF OPTICAL MEASUREMENTS

To obtain accurate and repeatable measurements of patterned media an optical system must have data which is both sensitive to structure dimensions and diverse enough to distinguish between different dimensions. Patterned media samples present some challenges to optical measurement systems because of the small width and height dimensions, which will decrease in future products, and the number of parameters of interest. For DTR and BPM templates and imprints, changes in width and height dimensions on the order of 10% of the nominal values result in only a fraction of a percent change in absolute reflectance at a given wavelength. This can lead to problems with both the noise level of the system and with parameter coupling, since the change of different parameters can have similar effects on the optical signal. Templates can be particularly challenging because of the low level of reflectance from the typical template structure, patterned fused silica. To meet the demands for DTR and BPM templates and imprints, an optical system must have multiple sets of unique spectra covering a large wavelength range with optimized signal to noise. Low system noise is necessary in order to distinguish changes in the measured data due to changes in structure dimensions (e.g. RLT or CD). The noise level of the system is critical to measurement repeatability, as changes in width and height of patterned media produce relatively small changes in the optical signal. The wavelength range and number of unique spectra relate to both repeatability and accuracy, as they determine how well different structure parameters can be separated. Variations of parameters of interest can cause changes anywhere in the measured spectrum, from the deep UV to the NIR, and for certain wavelength ranges, the change in two different parameters of interest can appear similar. Larger wavelength ranges and the use of multiple unique spectra, such as different polarizations of reflectance and transmittance, reduces parameter coupling. With data sensitivity and the ability to separate different measurement parameters, the measurement system still must be able to interpret the optical data and infer structure dimensions. This requires that the system include physically valid models that can adequately describe the optical signal based on the structure properties. The software must be able to overcome ambiguities in results and converge to a solution with the best fit between measured data and calculated data from the model. Fitting parameters without physical meaning should not be included as they can lead to inaccurate results.

3. n&k GEMINI SYSTEM

For the current study a commercially available spectrophotometer-based instrument was used, the n&k Gemini manufactured by n&k Technology, Inc. The n&k Gemini is a fully automated metrology system that uses polarized broadband reflectance and transmittance spectra, from 190nm to 1000nm in one nanometer intervals. The broadband light source consists of a tungsten filament lamp, providing visible and near-IR light, and a deuterium arc lamp, providing deep-UV radiation. The DUV-NIR signal is focused on the sample using reflective optics, in order to reduce the loss of light and enhance the signal to noise ratio. The transmittance and reflectance detectors consist of a holographic diffraction grating that separates the incident polychromatic beam into its constituent wavelength components and a photodiode array that records the raw data spectrum. The angle of incidence is 7º from normal and the measurement spot diameter is 50μm for both reflectance and transmittance. For samples that are partially transparent in the DUV-NIR this results in four unique measured spectra, two polarizations (within and perpendicular to the plane of incidence) of both reflectance and transmittance, named here as Rp, Rs, Tp, and Ts. For opaque samples, there is no transmittance and only the polarized reflectance spectra, Rp and Rs, are used for measurement. Figure 1 is a schematic of the n&k Gemini optical path.

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Figure 1. Optical path of the n&k Gemini Scatterometer

To extract values of n, k, film thickness and structure dimensions, a model of the measured area is created using analysis software. The software incorporates rigorously coupled wave analysis (RCWA) for periodic structures2, 3 with Forouhi-Bloomer dispersion relations relations4, 5 for n and k. The analysis model generates calculated reflectance and transmittance spectra using the nominal parameters and optimizes the values using nonlinear regression analysis, in order to minimize the difference between the measured and calculated spectra. Each parameter in the model can be either varied or fixed at a known value. The dimensions obtained upon the convergence with the highest obtainable goodness of fit are reported as measurement results. The n&k Gemini system is designed for both thin film and OCD measurements and can also be used to measure phase shift6, 7 and transmittance of transparent substrate samples. The measurement system requires a pad size of at least 60μm by 60μm. Individual features cannot be measured. Instead, a continuous array of periodic structures covering the 50μm diameter measurement spot is required, and the results obtained relate to the average structure values within the measured spot. Film thickness and n and k can be measured for thin film structures, and film thickness, etch depth, CD and sidewall angle can be measured for periodic 1D gratings and 2D hole arrays. The n and k spectra of materials, which can be determined from blank film measurements, are required as inputs to the 1D and 2D models, along with the structure pitch. The measurement system is equipped with an R-θ stage in order to orient DTR gratings perpendicular to the plane of incidence, as required by the calculation software. Although DTR structures actually consist of concentric circles around the disk center, they are modeled as line/space structures, considering that the radius of curvature is large relative to the spot size. The measurement method is non-contact, non-destructive and high throughput, with typical measurement times (including the move, acquire, measure, and analyze steps) on the order of 3s per measurement point.

4. PATTERENED MEDIA MEASUREMENT SENSITIVITY 4.1 Sensitivity Testing Method In order to test the feasibility of patterned media measurements using the n&k Gemini, we have performed simulations for a fused silica DTR template, a DTR imprint and a BPM imprint. Each structure is given nominal values of pitch, depth, CD and residual layer thickness. The calculated spectra comparing different values of depth, CD, and RLT demonstrate the sensitivity of the polarized reflectance and transmittance data (Rp, Rs, Tp, and Ts) from 190-1000nm to these measurement parameters.

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4.2 Fused Silica DTR Template Fused silica template simulations, calculated polarized reflectance and transmittance, for the structure in Figure 2 are shown in Figures 3 and 4. Differences in the spectra due to depth and CD changes of 5nm are on the order of 0.2% for both reflectance and transmittance. Rp and Tp are more sensitive than Rs and Ts to changes in the CD value, and the DUV wavelengths are most sensitive to changes in depth. The CD and depth have different effects on the data, considering all four spectra and the full wavelength range, so the parameters can be measured independently. The simulations show the importance of transmittance, and previous studies have demonstrated the improvement in measurement precision from the addition of polarized transmittance data for transparent substrate gratings8. The reflectance at both polarizations is less than 3% for the full wavelength range, while polarized transmittance is greater than 90%. With sensitivity to the depth and CD on similar levels for reflectance and transmittance, the transmittance data is less susceptible to system noise because of the high signal. The addition of the Tp and Ts spectra also doubles the amount of data available for analysis, reducing the coupling of parameters and further improving measurement repeatability.

Figure 2. DTR template with nominal dimensions for simulations, cross section (left) and top-down view (right)

Figure 3. Calculated polarized reflectance and transmittance (Rp, Rs, Tp, and Ts) for DTR templates with different depths, 100nm and 105nm

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Figure 4. Calculated polarized reflectance and transmittance (Rp, Rs, Tp, and Ts) for DTR templates with different CD values, 55nm and 60nm

4.3 DTR Imprint DTR imprint simulations for the structure in Figure 5 are shown in Figures 6-8. The reflectance is significantly higher than the template due to the materials and there is no transmittance. Differences in the spectra due to depth and CD changes of 5nm are on the order of 1%. Rp and Rs have similar sensitivity to the depth, CD and RLT values. The parameters have different effects on the data, so they can be measured independently. The 5nm change in depth and RLT appear to have similar effects on the spectra, but there are enough differences to separate the values. An increase in RLT of 5nm, for example, causes a shift up in Rp data in the 200-500nm wavelength range, while the 5nm increase in depth causes the Rp data to be lower at wavelengths near 300nm.

Figure 5. DTR Imprint with nominal dimensions for simulations, cross section (left) and top-down view (right)

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Figure 6. Calculated polarized reflectance (Rp and Rs) for DTR imprints with different depths, 75nm and 80nm

Figure 7. Calculated polarized reflectance (Rp and Rs) for DTR imprints with different RLT, 15nm and 20nm

Figure 8. Calculated polarized reflectance (Rp and Rs) for DTR imprints with different CD, 45nm and 50nm

4.4 BPM Imprint BPM imprint simulations for the nominal structure in Figure 9 are provided in Figures 10-12. The BPM data is different than DTR in that the Rp and Rs data are almost identical. This is due to the fact that the incident angle is near normal and the hole structure is symmetric in the x and y axis. If the holes were ovals with a difference in the x and y radius, there would be a larger separation between the Rp and Rs spectra. Observing this difference can provide a measure of the hole roundness, so long as the long and short axes are parallel or perpendicular to the plane of incidence. As with the DTR structure, the effects of changing RLT and depth appear to have similar effects on the reflectance spectra. There are, however, differences in the DUV behavior, allowing the parameters to be separated.

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Figure 9. BPM Imprint with nominal dimensions for simulations, cross section (left) and top-down view (right)

Figure 10. Calculated polarized reflectance (Rp and Rs) for BPM imprints with different depths, 65nm and 70nm

Figure 11. Calculated polarized reflectance (Rp and Rs) for BPM imprints with different RLT, 15nm and 20nm

Figure 12. Calculated polarized reflectance (Rp and Rs) for BPM imprints with different CD, 25nm and 30nm

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5. DTR IMPRINT MEASUREMENT AND COMPARISON TO SEM 5.1 DTR Imprint Sample

A patterned magnetic media sample was produced using a Nano Imprint Lithography tool from Obducat. The replication of the stamp onto magnetic media disks is performed employing a two-step imprint process. Step 1 is the generation of an intermediate polymer stamp (IPS®) and step two is the simultaneous thermal and UV (STU®) imprint process that replicates the IPS® onto the target substrate. IPS® and STU® are registered trademarks of Obducat. Both imprint tool and process are capable of high-volume manufacturing with a throughput of 600 HDD disks/hr. per lithography system.

An important aspect in the manufacturing of patterned magnetic media is the residual layer on the imprinted substrates. The residual layer is etched in the downstream processing after the NIL process. It is thus important to be able to measure the residual layer accurately and rapidly. For this purpose, a sample with residual layer significantly thicker than what is usual was produced for measurements.

5.2 n&k and X-SEM Measurement Methods

The DTR imprint sample was measured at 186 locations by the n&k Gemini and 4 locations by cross section SEM, shown in Figure 13. The cross section SEM measurements are ordered, one through four, from the inner to outer diameter. The n&k Gemini measurements taken closest to each X-SEM location are used for comparison. The nominal structure is displayed in Figure 14,

The sample was cut along the servo line for cross section SEM analysis. This, along with the use of relatively low voltage helped avoid damage to the resist. There are some expected errors due to the image resolution and the small feature size. The SEM values, determined by averaging the mid CD and depth of at least five features within each cross section image, are given an estimated error of +/- 5%. The reading of the RLT value with the SEM may not be accurate because the sample was cut along the servo line and view of the residual layer may be partially blocked. For this reason, RLT results are not compared with the cross section SEM.

Figure 14. Nominal DTR Structure Figure 13. Measurement Locations, X-SEM and n&k Gemini

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Figure 16. Disk Center Point n&k Measurement Result Figure 15. Disk Center Point SEM Image

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5.3 n&k and X-SEM Results Comparison The model for analysis of the polarized reflectance data was updated to match the profile from the SEM images. This single model was then used to fit the data from all n&k Gemini measurement locations, including those near the SEM measurements. The trench depth, RLT, and CD values were allowed to vary to obtain the best fit between the calculated and experimental data. There is good agreement between the SEM images and the resulting profile determined by the n&k Gemini at the closest locations. In Figures 15 and 16, the SEM image of the printed area at the third point from the disk inner diameter is shown, along with the n&k Gemini profile result, based on the best fit to polarized reflectance data at the nearest location. Comparing all four SEM measurement locations to the nearby scatterometer results, there is good agreement for both depth and CD, shown in Figures 17 and 18. While there is some ambiguity in the SEM results the trends are similar, with depth increasing from inner to outer diameter and CD decreasing.

5.4 n&k Uniformity Maps The uniformity maps, given for mid CD, depth, and RLT in Figures 19-21, are based on the compilation of individual point results. The results show that CD is highest at the inner diameter of the disk and decreases towards the outer diameter. The depth is highest where the RLT is lowest and vice versa. The range of both is similar, so the non-uniformity in RLT appears to be mainly due to the imprint process, not deposition of the resist layer. Both the depth and RLT are slightly lower than the nominal values. Dense uniformity maps like these can be measured optically in a matter of minutes, and the resulting information can be used to help control and improve patterned media processing.

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6. MEASUREMENT SENSITIVITY FOR FUTURE STRUCTURES While measurement capabilities for current templates and imprints can be tested directly, it is obviously more difficult to estimate measurement capabilities for structures that are not yet being produced. Future generation templates and imprints will be smaller than current structures in both width (pitch and CD) and height (trench depth). In order to estimate the feasibility of future scatterometry measurements, simulations are done for a range of structures that are similar to current templates and imprints but with smaller CD and depth values. Values of CD from 50nm to 5nm and

Figure 19. n&k CD (nm) Uniformity Map Figure 20. n&k Depth (nm) Uniformity Map

Figure 21. n&k RLT (nm) Uniformity Map

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Relative Sensitivity vs. Template Depth

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pitch from 100nm to 10nm are tested, with the CD set to half of the pitch. Depth is tested from 100nm to 5nm. To quantify the measurement sensitivity, the calculated spectra (Rs, Rp, Ts, and Tp from 190-1000nm) of a structure is compared to calculated spectra of a similar structure, with a change in either depth or CD of 1nm. The difference between the spectra at all wavelengths is added together and used as a relative measure of the sensitivity. The testing is performed for DTR templates and imprints and a BPM imprint. Using this method, measurement sensitivity of smaller structures can be compared to current sized structures for both CD and depth. The sensitivity relates to the measurement accuracy and repeatability and gives an indication of measurement capabilities without the need for actual samples. For fused silica templates, the sensitivity to depth, shown in Figure 22, remains fairly constant until the depth is less than 20nm, where there is a drop off in sensitivity. For templates of this size, the measurement will be less repeatable than current sized structures. Sensitivity to the template CD, shown in Figure 23, continuously increases as CD and pitch become smaller. In other words, a change in CD of 1nm has a much greater effect on a structure with pitch of 20nm and CD of 10nm than a structure with pitch of 80nm and CD of 40nm. The simulations show that sensitivity to CD relates to the percent change in the CD value relative to the structure pitch. A 1nm change of CD will affect the spectra of a 20 nm pitch template by approximately the same amount as a change of 2nm for a 40nm pitch template and 4nm for an 80nm pitch template. Therefore, as template pitch decreases for future structures, the sensitivity to CD will increase by the same factor.

The measurement sensitivity to depth for DTR and BPM imprints increases slightly as the depth decreases from 100 to 5nm, as shown in Figures 24 and 26. There is not a drop off in sensitivity as the depth approaches zero like the DTR template. Sensitivity to DTR and BPM imprint CD, shown in Figures 25 and 27, increases as the pitch becomes smaller, similar to the template CD. Comparing the measurement sensitivity of the DTR template to DTR and BPM imprints at a given set of depth, CD and pitch values, the imprints are two to three times more sensitive to changes in depth and CD.

Figure 22. Future Sensitivity to Template Depth Figure 23. Future Sensitivity to Template CD

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Relative Sensitivity vs. DTR CD

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Considering the sensitivity to changes in CD and depth of templates and imprints, the measurement precision for future structures is expected to remain steady or improve. Other structure details, including material properties and film thickness of layers below the imprinted area can influence the measurement sensitivity and should also be considered in evaluating specific measurements. With other structure properties constant, however, the CD measurement sensitivity is inversely proportional to pitch. While other metrology techniques can suffer from decreasing feature size, the polarized reflectance and transmittance based scatterometer will have improved precision for CD measurements.

7. SUMMARY The polarized reflectance and transmittance based scatterometer, the n&k Gemini, has sensitivity to the measurement parameters for DTR templates, DTR imprints, and BPM imprints. The large wavelength range and optimized signal to noise for four unique spectra (Rp, Rs, Tp, and Ts) provide the necessary data to measure depth, CD, and RLT values simultaneously. A measurement of a DTR imprint shows good correlation with cross section SEM values for both depth and CD, and uniformity maps reveal information about the imprint process. With templates and imprints of decreasing size showing similar sensitivity for depth and greatly increased sensitivity to CD over current structures, this optical technique can provide fast, non-destructive measurements for the development and production of current and future patterned media.

Figure 26. Future Sensitivity to BPM Imprint Depth Figure 27. Future Sensitivity to BPM Imprint CD

Figure 24. Future Sensitivity to DTR Imprint Depth Figure 25. Future Sensitivity to DTR Imprint CD

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8. REFERENCES [1] Kryder, M. H., Carnegie Mellon U., “Magnetic Recording Roadmap,” Technical Symposium IDEMA (2007). [2] Lam, J. C., Gray, A., Howell, R., and Chen, S., “Polarized transmittance-reflectance scatterometry measurements of

2D trench dimensions on phase-shift masks,” Photomask Japan 2007 – Proc. SPIE 6607, 660710 (2007). [3] Moharam, M. G., Grann, E. B., Pommet, D. A., Gaylord, T. K., “Formulation for stable and efficient

implementation of the rigorous coupled-wave analysis of binary gratings,” J. Opt. Soc. Am. 12, 5 (1995). [4] Forouhi, A. R. and Bloomer, I., “Optical Dispersion Relations for Amorphous Semiconductors and Amorphous

Dielectrics,” Physical Review B 34, 7018 (1986). [5] Forouhi, A. R. and Bloomer, I., “Optical Properties of Crystalline Semiconductors and Dielectrics,” Physical

Review B 38, 1865 (1988). [6] Callegari and Babich , K., “Optical Characterization of Attenuated Phase Shifters,” Proc. SPIE 3050, 507 (1997). [7] Brooks, C. B., Buie, M. J., Waheed, N. L., Martin, P. M., Walsh, P., and Evans, G., “Process Monitoring of Etched

Quartz Phase Shift Reticles,” Proc. SPIE 4889, 25 (2002) [8] Gray, A., Lam, J. C., Chen, S, “Innovative application of the RCWA method for the ultra-sensitive transmittance-

based CD measurements on phase-shift masks,” Proc. SPIE 6533, 65330F (2007)

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