New SEMI Standard for Density and Porosity of the Chemical Mechanical Planarization Pads
By Alexander Tregub, Leader of the SEMI Liquid Chemicals North America Chemical Mechanical Planarization Consumables Task Force, Intel Corp., and Laura Nguyen, SEMI
Chemical-mechanical planarization (CMP) is an important step in IC manufacturing that is required to achieve global and local planarity for subsequent lithography processes. CMP has been one of the fastest-growing segments of the semiconductor industry for the past 20 years.
The CMP process is highly critical for achieving sustainable yields in IC manufacturing. This area is also one of the most difficult to control. Quality issues in the CMP process are extremely expensive and can have a significant impact to device yields and reliability. As such, reducing the variations in CMP pads properties through standardized reporting of the pad parameters, predictive of fab processes, has become a high priority for advanced IC manufacturing nodes.
Most of industrial CMP pads are intentionally manufactured with built-in porosity [Ref.1]. Pad pores serve as micro reservoirs for holding slurry during CMP process, thus, increasing the efficiency of planarization and reducing slurry consumption. Accurate measurements and reporting of density of porous pads and pad porosity is one of the prime challenges of quality control (QC) for CMP pads.
In this article, we will review optimization of the measurements and reporting of CMP pad density and porosity, the important quality parameters that may determine removal rate, defectivity, dishing and erosion [Ref.2,3], and other important key performance Indicators (KPI) of the layers being planarized.
Pad Density
Pad density, D, is frequently reported as the ratio of the pad weight, W, to calculated geometric pad volume, V, using simple equation (1)
D = W/V (1)
where D is calculated pad density, W is measured weight of the pad or pad coupon, V is the calculated geometric volume of the pad or pad coupon.
However, the simple way of reporting pad density per equation (1) is not accurate, as it does not take into account the contribution of the pad pores. Reporting pad density per the simple equation in (1) results in a quality parameter that is both inaccurate and non-predictive of pad performance.
Pad Porosity
Pad porosity is frequently reported on quality documents as pore count measured across the limited pad areas using optical microscopy, Scanning Electronic Microscopy (SEM), or Transmission Electronic Microscopy (TEM). However, this approach does not take into account contributions of pore shapes, sizes, and, most importantly, pore volumes. Reporting pad porosity as pore count across the limited area of a pad coupon results in a quality parameter that is non-repeatable/not-reproducible, and does not reflect contribution of all pores into pad performance.
Outdated and inconsistent metrologies employed to measure and report pad density and porosity result in quality issues and reduced yield in the Fabs, consumable rejects, availability issues, and lack of correlation between reported pad parameters and fab performance. Until now, there have been no industry-wide consensus Standards for reporting pad density and porosity.
To close this gap, a group of industry CMP experts partnered with the SEMI International Standards Program, to utilize its reputation for bringing together industry stakeholders to work towards developing globally accepted technical standards, to develop the first suite of SEMI Standards focused on CMP consumables metrology. To date, SEMI Standards for CMP pad hardness (SEMI C100), CMP pad density and porosity (SEMI C102), CMP pad windows, and CMP pad conditioners (SEMI C103) have been published or are in the publication process.
The Chemical Mechanical Planarization Consumables (CMP-C) Task Force under the Liquid Chemicals Global Technical Committee within the SEMI Standards Program has developed the SEMI Standard SEMI C102, Guide for Reporting Density and Porosity of Chemical Mechanical Planarization (CMP) Pads used in Semiconductor Manufacturing.
Per the new Guide, to account for pore contribution into pad density, QC procedures should be based on such standard test methods, as liquid displacement [Ref.4], or pycnometer [Ref.5].
For pad porosity, the new Guide recommends using QC tests based on liquid porosimetry [Ref.6] or gas pycnometer [Ref.7]. Liquid porosimetry or gas pycnometer methods allows measuring and reporting total pore volume (TPV) of all pores of different sizes, shapes, and volumes, which are present in the pads. In addition, mercury porosimetry tools may have high volume chambers to accommodate large portions of the pads, including the whole pads for 300 mm wafer polishing.
In addition, nanoindentation or nanoDMA metrologies can be used to assess porosity of CMP pads [Ref.8]
Table 1 summarizes existing gaps in measuring and reporting out parameters for CMP pad density and porosity and explains, how the gaps have been closed with introduction of SEMI C102.
|
Before SEMI Standard |
After SEMI Standard |
|
Legacy density metrologies do not take into account pad pores |
Recommended metrologies: mercury porosimetry, gas pycnometer, nanoindentation/nanoDMA accounts for pore contributions |
|
Legacy pore count metrologies do not take into account differences in pore sizes, shapes, and volumes |
Mercury/liquid porosimetry and/or gas pycnometer measure Total Pore Volume (TPV) of pores of all sizes, shapes, and volumes |
|
Legacy pore count metrology assesses only small areas of the pads |
Mercury/liquid porosimetry, gas pycnometer can measure TPV for large pad samples or whole pads |
Table 1. Closing the gaps in measuring and reporting of CMP pads density and porosity with introduction of SEMI C102
The new guide encourages CMP pad suppliers to continue reporting legacy parameters to enable continuation of comparisons to the historic databases.
Measuring and reporting density and porosity of CMP pads in accordance with SEMI Standards will benefit suppliers of CMP pads by better describing the advantages of the supplier’s new products, and by reducing the risk of quality issues at the fabs of the supplier’s customers.
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References
- A. Prasad, G. Fotou, Shoutian Li. The Effect of Polymer Hardness, Pore Size and Porosity on the Performance of Thermoplastic Polyurethane (TPU) Based Chemical Mechanical Polishing (CMP) Pads, www.cabot.com, 2014
- A. Tregub. Characterization of Solid Consumables for CMP: Unknown Known or Known Unknowns?: State-of-the- Art CMP Short Course “Characterization of CMP consumables: correlation between material properties and CMP performance”, Twelve International Conference for Using ULSI Multilevel Interconnection, CMP Planarization, March 5, 2007
- A. Tregub, T. Bramblett, S. Narayanan. Characterization of CMP consumables: correlation between material properties and CMP performance. Twelve International Conference on Chemical-Mechanical Polish, March 5, 2007, Fremont, California
- ASTM D792-13. Standard test method for Density and Specific Gravity (relative Density) of Plastics by Displacement
- ASTM D854. Standard Test Methods for Specific gravity of Soil Solids by Water Pycnometer
- Carlos A. Leon y Leon. New perspectives in mercury porosimetry, Advances in Colloid and Interface Science 76-77 (1998) 341 – 372
- A. Obaton et. al. Investigation of new volumetric non-destructive techniques to characterize additive manufacturing parts. Weld World 62 (2018), pp 1049-1057
- CMP pads Characterization by Statistical Nanoindentation, BRUKER, Technical Note, September 1, 2020
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June 15, 2022