In-situ metrology for pad
surface monitoring in CMP■ The CMP process
Chemical Mechanical Planarization (CMP) is one of the most critical processes
in the semiconductor, hard disk and LED wafer manufacturing segments.
The process is applied to assure the required planarity of the substrate wafer
and for re-planarization at some intermediate points following depositionand lithographic processing for the structures built upon the wafer. In
general, planarization is necessary to ensure functionality of the mutli-levelinterconnects within the structures, but the technique is also commonlyused to reduce wafer thickness whilst maintaining uniformity.
During the CMP process, the wafer is held on a rotating fixture and pressedagainst a rotating polishing pad while an abrasive chemical liquid (slurry) is
distributed between the wafer and the pad. Pad rotation and concentric
grooves engineered into the pad surface serve to transport the slurry across theinterface between the pad and wafer. The chemical slurry weakens the surfaceof the wafer, thus enabling removal of material by asperities of the pad [1].
The quantity of material removed from the wafer and the quality of theoverall process is highly influenced by the surface properties of the polishingpad. The surface of the pad degrades as a result of the polishing action, soit needs to be constantly reconditioned. Conditioning involves an abrasive
process on the pad surface, where a rotating abrasive or conditioning disk
recovers the pad surface. These disks are usually made of stainless steel orelectroplated diamond.
CASE STUDY IN-SITU METROLOGY FOR PAD SURFACE MONITORING IN CMP

CMP process schematic diagramabWithout
Wafer CMP
manufacturing With CMPwithout CMP (a) and with CMP (b)1

■ Surface Metrology for CMP
The need for surface metrology for the CMP process is evident given thatmechanical interaction between the surfaces is a critical variable for all
stages of the process. The surfaces that need to be periodically characterizedthroughout the process include the conditioning disk surface, the wafer
surface and the pad surface. In high volume production environments,
however, non-destructive in-situ pad characterization at natural pausesin the polishing process (for example, when changing wafers) is desired.
This would permit detection of drifts in key pad parameters and assist in
validation of process changes. The goal in all cases is to extend the lifetimeof consumables and to improve the yield of the whole process.

3D topography of conditioning disk surface

There are two main factors affecting pad surface degradation and lifecycle –pad groove occlusion and pad glazing.
Groove occlusion
During the polishing process the material removed from the wafer deposits
in the pad grooves, thus occluding them. This prevents uniform slurry
distribution across the water, thus causing non-uniform removal between
the center and edge of the wafer. Monitoring of groove occlusion enables
prediction of the need to clean the pad grooves and determining the best
point in time to do it. This cleaning operation can extend pad lifetime by up
to 20% [2].
Immersion objective2

Pad glazing
Glazing is a more complex phenomenon where the polishing ability of thepad is reduced due to surface degradation. This phenomenon enhances
wear between wafer and pad, thus increasing process temperature and that
may result in material selectivity during polishing. The phenomenon cannotCASE STUDY IN-SITU METROLOGY FOR PAD SURFACE MONITORING IN CMP

be predicted as easily as groove occlusion and requires constant monitoringin order to guarantee ideal performance of the CMP process.
In order to enable in-situ pad surface monitoring, the metrology approach
used must be able to work under wet conditions. Only immersion metrology
can meet these requirements. The main benefit of this approach is thatthe pad does not need to be removed from the polisher in order to becharacterized. This makes it possible to monitor pad glazing and grooveocclusion in-situ at various points in the pad’s life-cycle. In-situ metrology
has proven to extend pad lifetime, allowing operators to utilize the pads tothe end of their useful life.

■ A new immersion metrology system
Working closely with experts in the field, Sensofar has developed the mostsuitable solution to solve this problem. The goal: change the polishing pads
only when there is an absolute need, thus increasing yield per pad andminimizing downtime of the polishing systems.
The proposed solution is a non-destructive, in-situ metrology system, S mart
CMP. Utilizing a portable stand, the sensor can be simply placed on the padwhile the pad is still installed inside the polishing system and provides animmediate status on pad condition. S mart CMP is equipped with a laptop
computer for enhanced portability and has been designed to work both asa stand-alone sensor and alternatively as a system that can be integrated
into the production line as an automatic metrology solution. In both cases,
S mart CMP is capable of rapidly acquiring and analyzing data in order toeffectively monitor key pad characteristics.

S mart CMP, 3D optical sensor

This solution has already been proven to successfully monitor pad glazingand groove occlusion. Thanks to S mart CMP it has been clearly demonstrated
that CMP pads are largely under-utilized and are often discarded with morethan half of the useful lifetime still remaining.
CamerabBrightness

aFocus position (µm)(a) Optical diagram of S mart sensor; (b) Pixel axial responser

CASE STUDY IN-SITU METROLOGY FOR PAD SURFACE MONITORING IN CMP3■ Basic operation of S mart CMP
S mart CMP is provided with Sensofar patented microdisplay technology
and a high intensity blue LED, allowing combination of Confocal microscopy
and Focus Variation technologies within a single sensor [3]. A software plugin built around the CMP application provides all of the necessary tools andanalysis for this application within the S mart CMP control interface.
The microdisplay enables projection of different fringe patterns onto
the surface to be inspected, generating a confocal image of the surface.
The confocal image is a greyscale map of the surface, where pixels havebrighter signal if they are in the focus plane, while those out of focus appear
completely black. By performing a vertical scan on the surface, it is possible
to collect a stack of confocal images and obtain an axial response of thefocus for each pixel. Processing the axial responses makes it possible togenerate a 3D reconstruction of the surface.
By combining this metrology approach with an appropriate immersion
objective, the S mart sensor can measure the pad asperities while the pad isstill on the polisher.
Using the same immersion objective with Focus Variation technology,groove depth and width can be quickly determined in order to detect andmonitor groove occlusion.

■ Groove occlusion monitoring
Provided with Focus Variation technology, S mart CMP is able to measure a
pad groove within a few seconds. This allows characterization of the groove
width and depth, enabling groove occlusion monitoring along the process.
Automatic analysis is made possible for production environments thanks toa function within the CMP software plug-in that determines both the widthand depth of the groove independently of groove orientation.

Evolution of pad groove width and depth along 12 hour CMP processing■ Monitoring of pad glazing
Thanks to microdisplay Confocal microscopy, adapted to work in water
immersion conditions, S mart CMP is also able to characterize pad asperity
4

CASE STUDY IN-SITU METROLOGY FOR PAD SURFACE MONITORING IN CMP

following conditioning and polishing phases. This not only allows monitoring
and thus determination of the right point in time for conditioning, butalso permits characterization of the optimal conditioning time required toregenerate the pad surface. All these aspects help to extend usable (andvaluable) pad lifetime, reduce the need for control wafers and additionallyoptimize the process – thus also avoiding re-processing of otherwise
completed wafers.
3D measurement of a new pad groove

Image showing evolution of the pad asperity height during the polishingprocess. After several hours of polishing a ‘glazing peak’ appears in thesurface distribution. By monitoring the pad surface at intervals during thepolishing process, the pad surface can be duly conditioned (intervention) inorder to revert the surface condition to the initial state.
glazing peak

2 hr

4 hr

6 hr

interventio

Image showing comparison for the pad before intervention (black) vs thepad following intervention (red). The image on the left side was for thesurface after 6 hour polishing process and the right one is after 12 hours.
– No Intervention– Intervention– No Intervention– Intervention

6 hour12 houraNew padb

Good pad- 10h usec

Bad pad - 3h use(a) New pad; (b) Good pad after 10h use; (c) Bad pad after 3h use

CASE STUDY IN-SITU METROLOGY FOR PAD SURFACE MONITORING IN CMP5■ Automatic analysis
To simplify surface parameter control, the analysis of the acquired data canbe automated. The operator then simply places the sensor over the padsurface and focuses on it, and once the acquisition has started the software
will automatically show the target parameter value.
Special analysis algorithms developed for this application can automaticallydetect groove width and depth to monitor groove occlusion, while a different
algorithm is designed to analyze the critical parameters of the asperities on
the pad surface in order to detect pad glazing.

6CASE STUDY IN-SITU METROLOGY FOR PAD SURFACE MONITORING IN CMP

■ References
1 J. McGrath, C. Davis. Polishing pad surface characterization in chemicalmechanical planarization. Journal of material processing technology, 153-154(2014).
2 T. Moore, N. Schwarz. NEOX Ex-Situ CMP Pad. NCCAVS, CMP Users Group
Proceedings (2013).
3 R. Artigas, F. Laguarta, C. Cadevall. Dual-technology optical sensor head for 3D
surface shape measurements on the micro- and nanoscale. Optical Metrology inProduction Engineering, Proc. SPIE 5457 (2004).

TECHNICAL SPECIFICATIONSSENSORDimensionsOBJECTIVE
WeightMagnification

26x28x37cm

9 Kg

20X

10.2x11x14.6 “

19,8 Ibs(Immersion)

CONFOCAL

FOCUS VARIATION

Working

Field ofOptical

VerticalMeasurement

VerticalMeasurement

Distance (mm)View (µm)

Resolution (µm)

Resolution (nm)Time (s)Resolution (nm)Time (s)2.00

877x660

0.31

2020155

CASE STUDY IN-SITU METROLOGY FOR PAD SURFACE MONITORING IN CMP

7SENSOFAR is a leading-edge technology
company that has the highest quality standards
within the field of surface metrology
Sensofar Metrology provides high-accuracy optical profilers
based on confocal, interferometry and focus variation
techniques, from standard setups for R&D and quality
inspection laboratories to complete non-contact metrology
solutions for in-line production processes. The Sensofar Grouphas its headquarters in Barcelona, also known as a technology
and innovation hub in Europe. The Group is represented in over
30 countries through a global network of partners and has its own
offices in Asia, Germany and the United States.
HEADQUARTERS

SENSOFAR METROLOGY | BARCELONA (Spain) | T. +34 93 700 14 92 | info@sensofar.com
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sensofar.com

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