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Seeing Through the Haze Process Monitoring and Qualification Using Comprehensive Surface Data Frank Holsteyns, Jan Roels, Quoc Toan Le, Karine Kenis, Paul W. Mertens, IMEC
Haze, the low frequency signal of light scattering on unpatterned wafer, is already accessible on the Surfscan SP1 in most fabs and contains very useful surface information. It can be used as a proxy for other metrology tools to measure thickness, reflectivity, roughness, defects, and can be integrated in SPC tests for equipment monitoring. When combined with defect measurements, it provides additional wafer surface information unrevealed by defect monitoring alone.
Introduction Max LPD
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Haze, the low frequency signal of light scattering on unpatterned wafers is a common technique used in semiconductor production fabs to monitor and qualify a variety of process steps. The information distilled out of the captured scattering light is two-fold: 1) scattering events (light point defects) used to count and size particles, and 2) haze, or the background scattering signal (Figure 1). This study shows that haze, collected as the low frequency signal from a darkfield inspection tool, is a very effective way to gather surface information. The analysis of the haze data can be used as a powerful tool to evaluate process performance concerning defectivity, uniformity, roughness, sealing, etc.
{ Actual Haze
Figure 1. The captured scattered light consists of light point defects (LPD) and the haze signal.
Light scattering tools
In this study, we focus on the use of the KLA-Tencor Surfscan SP1DLS unpatterned wafer inspection tool to extract the haze signal from the tool’s scattered signal. The wafer is illuminated by a laser, with a beam either normal to the wafer surface or oblique (70° to the normal). The scattered light, coming from different defects as well as the background of the wafer surface, is collected 50
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by two separate channels. The low frequency component of the signal of these channels is given as the haze signal, expressed as scattering power fraction per area of detector to the total incident power (ppm/Star radial).
Haze reference wafers To validate the haze signal, a set of standard calibration wafers was developed on a SEZ spin processor. These wafers, which remained stable over time and were
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Monitoring with haze The haze signal monitoring capability on KLA-Tencorâ&#x20AC;&#x2122;s Surfscan SP1DLS unpatterned wafer inspection tool can be used in many ways: as a measure of nano-sized particles (<50 nm) density, surface roughness of metallic layers, grain size variations, and to inspect small process defects induced in CMP and deposition processes The new MX 4.0 haze normalization option further allows normalized haze data to be used for thorough monitoring of process excursions in wafer and IC fabs as the tool inspects wafers and captures defects. Fluctuations in process parameters, such as temperature and pressure, often cause deviations in poly grain size during the poly gate process, one of the most critical steps in DRAM device production. These deviations can lead to lower-binning products or device failures. One example of haze monitoring of grain size variation for a rugged poly process is shown below:
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Haze Monitor
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cleanable, allowed us to evaluate tool stability and to perform tool-to-tool comparisons for different light scattering tools. As shown in Figure 2, a set of different standard haze wafers is used as a reference to monitor the haze signal over time for the different channels of the SP1 (in this case the darkfield wide oblique channel). A good reference is necessary, since a shift in haze value was observed after a technical intervention with the inspection tool.
Normalized haze measurement can be used for poly furnace pro-
Avg. Haze [ppm/Sr]
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The creation of these different haze standards, using varying surface roughnesses, allows us to link AFM measurements (RMS (nm)) with haze (ppm/Sr). Figure 3 demonstrates that most of the SP1 channels are very sensitive for changes in roughness, especially for wafers with a particular frequency diagram.
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Figure 3. Relation between AFM (RMS) and SP1 (Haze) measurements for five different wafers.
The creation of these standard haze wafers provided the opportunity to integrate the haze data as an evaluation tool in the statistical process control (SPC) methods, enabling process excursions to be flagged. Process monitoring and qualification
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it is important that, in addition to light point defects (LPDs), the haze data in defect detection is taken into account to understand and monitor a broader spectrum of process-induced variations.
tool), absolute reflectivity (using the FT-700 tool), and sheet resistance (using the RS75 resistivity measurement tool). From an earlier study,1 the relation between reflectivity and haze was shown. This makes it possible to replace the 49-point reflectivity measurement (Figure 4), of the FT-700 and replace it with a full wafer plot from the SP1. By doing so, this measurement â&#x20AC;&#x201D; using oblique incident beam and s-s-s polarization (for beam and two detectors) â&#x20AC;&#x201D; can be combined with the particle measurement. An upper spec limit for haze can be determined (using the maximum reflectivity laid down by the tool manufacturer) and set for the process (Figure 5). A sequence of variations observed in the final haze value (but still in spec) of the W could be related to the use of reclaim wafers of lesser quality. The excursion was due to a variation in the process conditions: a fluctuation in temperature).
CVD and PVD processes
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Figure 4. Output file haze measurement (A) and a reflectivity measurement (B).
For CVD and PVD processes, the surface roughness and spatial frequency of metallic layers like tungsten (W) and tantalum/tantalum nitride (Ta/TaN), and the thickness uniformity and roughness for transparent layers like silicon oxynitride (SiO2), low-k and photoresist (typically spin-on processes), can be evaluated using haze data. Thickness information can be extracted because the standing waves in the metallic layer cause different laser light intensities at the top layer. The haze signal can be related to other metrology data such as reflectivity, thickness and sheet resistance. The benefits of using the haze signal as a proxy for other metrology steps are that it can be combined with particle measurements, it has a very short process time that reduces overall inspection time, and a full wafer map is provided.
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Figure 5. Weekly monitoring of the W-CVD process in the prototyping line.
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The study of the Ta/TaN layers2 focuses more on the sensitivity of the detectors (varying sensitivities for different scattering angles) for different spatial frequencies (see the power spectral density diagram) of the wafer surface. The higher the spatial frequency, the more light will be scattered to the wide detector. The ratio of the detectors to the frequency of the wafer surfaces provides information about the overall morphology of the surface under consideration. Therefore, we could conclude that a deposition process with a higher bias provides smoother surfaces (Figure 6). AFM images confirm these findings,for the 50-50 and the 128-128 bias setting (biasTaN-biasTa). Similarly, the copper-seed layer can be evaluated, and the self-anneal process for the different stacks can be monitored by the haze channel of the SP1. Control of this self anneal process plays a role in the suppression of defects like void formation during plating. Haze information for the barrier and copper-seed layers allows us to select the optimum material for the process, as well as to set specs for equipment SPC tests.
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Oxide deposition Oxide deposition processes can be monitored for uniformity or roughness by the haze channel. Both measurements are determined by polarizing the light in different ways. The uniformity can be evaluated by the S-polarization. The SP1 haze plot can be compared with the thickness plot of the KLA-Tencor SpectraFx thin film metrology tool (Figure 7). Any surface uniformity problem can be detected by the SP1, but not quantified. The surface roughness and also possible defect regions can be evaluated by C-polarization. The
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Daily Monitoring Figure 8. SPC tests for 200 nm oxide using haze and the LPD channel. Cooling problems are monitored with haze in five cases, and with the 0.10
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Figure 6. HF/LF ratio for different Ta/TaN barriers and AFM measurements. Higher bias provides a smoother surface.
SPC tests for particles can be extended to the measurement of haze and particles. As shown in Figure 8, the role of haze can play an important role in identifying tool defects; in this particular case, wafer cooling problems. It is interesting to note that, in all cases, haze data indicates process problems, unlike the LPD channel.
Nano-particles The industry is facing new challenges concerning particle removal beyond the resolution of the state of the art light scattering tools (φLSE <50 nm). The limitations of
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being able to measure wafers with small particles and/or in high densities can be overcome at the full-wafer level by using haze. This method3 can be used to evaluate performance and uniformity for cleaning tools like megasonic cleaners (Figure 9). A lot of 30 nm particles are not removed by megasonics due to the shadowing effect of the wafer carrier. Based on the Raleigh approximation, a simple model is developed to describe the added haze of a wafer ηadd due to a higher density of particles σ.
ηadd = Ωtool • Πpart • σ In this equation, Ωtool stands for the tool constant (influenced by laser-light-wavelength and optical characteristics) and Πpart for the particle constant (influenced by parameters like diameter and refractive index). The added haze increases proportionally to the particle
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Figure 7. Output file of (A) a haze measurement and (B) a thickness
Figure 9. An evaluation of a megasonic cleaning tool for 30 nm SiO 2
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particles. The SEM review tool is used to count particles.
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3000 Defects > 0.2 µm
2500 2000 1500 1000 500 0 -500 25 Figure 11. On the left, a haze plot after toluene treatment; on the
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Figure 10. SPC tests for a batch stripper, haze and LPDs are given, in
right, a decorated defect which caused the sealing problem.
dielectrics, after CMP, treated with toluene and measured with the SP1. This haze data is analysed using the MX4.0 haze analysis capability of the SP1. The results reveal the easily-neglected condition that a clear sealing is lacking at the edge of the wafer and at certain points on the wafer surface, due to the presence of big particles.
some cases only haze is measured.
density. Using SEM review, a clear relationship can be then established between haze and particle density.
Cleaning performance A spray acid tool (batch stripper) is used for chemical cleaning, stripping and etching. For the daily SPC tests, a blank silicon wafer is processed with H2SO4/H2O2 (10:1 ratio) for 10 minutes at 95 degrees C. In addition to detecting traditional particle defects, haze is useful for identifying other process problems. For example, when the rinse step following the acid step is not performing well, sulfuric residues will remain on the wafer surface and form ammonium sulfate crystals.4 The haze signal will be high because of the presence of these crystals. Figure 10 illustrates that the haze signal is too high with respect to the target value set for the Si wafers (0.05 ppm). To detect these problems and test the effectiveness of the cleaning process, the haze data is necessary, and can be simultaneously used with the LPD data.
Sealing defects The combination of solvent adsorption and haze allows sealing defects at the surface (BEOL) to be localized and quantified. Results on different hardmasks (HM)/dielectric stacks, such as CVD HM on CVD dielectric, CVD HM on spin-on dielectric, and spin-on HM on spin-on dielectric, can be easily evaluated. Figure 11 provides an example of a CVD HM on CVD
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Aknowledgemens
The author would like to thank the Surfscan Division of KLA-Tencor for the fruitful and interesting cooperation in this study, and SEZ for the creation of the haze reference wafers. This article was originally presented at the 2003 ISSM Conference. Proceedings of the 2003 IEEE International Symposium on Semiconductor Manufacturing, September 30 to October 2, San Jose, California, USA. Refernces 1. F. Holsteyns et al., “Process Monitoring and qualification of CVD/PVD tools using comprehensive surface haze information,” presented at the KLA-Tencor Yield Management Seminar, Japan, 2002. 2. L. Carbonell et al., “Defectivity study of Cu Metallization Process by Dark- and Bright-Field inspection,” Solid State Phenomena, vol. 92, pp 281-286, 2003. 3. K. Xu et al., “Relation between Particle Density and Haze on a Wafer: a new approach to measuring Nano-Sized Particles,” Solid State Phenomena, vol. 92, pp 161-164, 2003. 4. A.L.P. Rotondaro et al., “Interaction of Sulphuric Acid Hydrogen Peroxide Mixture with Silicon Surfaces,” Proceedings of the 2nd international symposium on ultraclean processing of silicon surfaces, pp 301-304, 1994.