Application note

In-situ metrology for pad
surface monitoring in CMPThe CMP process
Chemical Mechanical Planarization (CMP) is one of the mostcritical processes in the semiconductor, hard disk and LED wafer
manufacturing segments. The process is applied to assure therequired planarity of the substrate wafer and for re-planarization
at some intermediate points following deposition and lithographic
processing for the structures built upon the wafer. In general,
planarization is necessary to ensure functionality of the mutlilevel interconnects within the structures, but the technique is alsocommonly used to reduce wafer thickness whilst maintaining
uniformity.

CMP process schematic diagram

During the CMP process, the wafer is held on a rotating fixture andpressed against a rotating polishing pad while an abrasive chemicalliquid (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 the interface between the padand wafer. The chemical slurry weakens the surface of the wafer,thus enabling removal of material by asperities of the pad [1].
The quantity of material removed from the wafer and the qualityof the overall process is highly influenced by the surface propertiesof the polishing pad. The surface of the pad degrades as a result of
the polishing action, so it needs to be constantly reconditioned.
Conditioning involves an abrasive process on the pad surface, wherea rotating abrasive or conditioning disk recovers the pad surface.
These disks are usually made of stainless steel or electroplated
diamond.
In-situ metrology for pad monitoring in CMP(a)Without
CMP

With(b)CMP

Wafer manufacturing without CMP (a) and with CMP (b)1Surface Metrology for CMP
The need for surface metrology for the CMP process is evident
given that mechanical interaction between the surfaces is a criticalvariable for all stages of the process. The surfaces that need to beperiodically characterized throughout the process include theconditioning disk surface, the wafer surface and the pad surface. Inhigh volume production environments, however, non-destructive
in-situ pad characterization at natural pauses in the polishing process(for example, when changing wafers) is desired. This would permitdetection of drifts in key pad parameters and assist in validation ofprocess changes. The goal in all cases is to extend the lifetime ofconsumables and to improve the yield of the whole process.

3D topography of conditioning disk surface

There are two main factors affecting pad surface degradation andlifecycle – pad groove occlusion and pad glazing.

Groove occlusion

During the polishing process the material removed from thewafer 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 toclean the pad grooves and determining the best point in time to doit. This cleaning operation can extend pad lifetime by up to 20% [2].

Pad glazing

Glazing is a more complex phenomenon where the polishing abilityof the pad is reduced due to surface degradation. This phenomenon
In-situ metrology for pad monitoring in CMP

Immersion objective2

enhances wear between wafer and pad, thus increasing processtemperature and that may result in material selectivity during
polishing. The phenomenon cannot be predicted as easily as groove
occlusion and requires constant monitoring in 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 that the pad does not need to be
removed from the polisher in order to be characterized. This makes
it possible to monitor pad glazing and groove occlusion in-situ atvarious points in the pad’s life-cycle. In-situ metrology has provento extend pad lifetime, allowing operators to utilize the pads to theend of their useful life.

A new immersion metrology system
Working closely with experts in the field, Sensofar has developed themost suitable solution to solve this problem. The goal: change thepolishing pads only when there is an absolute need, thus increasing
yield per pad and minimizing 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 pad while the pad is still installed inside the polishingsystem and provides an immediate status on pad condition. S mart
CMP is equipped with a laptop computer for enhanced portability
and has been designed to work both as a stand-alone sensor andalternatively 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 to effectively
monitor key pad characteristics.
This solution has already been proven to successfully monitor padglazing and groove occlusion. Thanks to S mart CMP it has beenclearly demonstrated that CMP pads are largely under-utilized andare often discarded with more than half of the useful lifetime still
remaining.

S mart CMP, 3D optical sensor

Brightness

Camera

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

In-situ metrology for pad monitoring in CMP

3Basic operation of S mart CMP

S mart CMP is provided with Sensofar patented microdisplay
technology and a high intensity blue LED, allowing combinationof Confocal microscopy and Focus Variation technologies within
a single sensor [3]. A software plug-in built around the CMP
application provides all of the necessary tools and analysis for thisapplication 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 thesurface. The confocal image is a greyscale map of the surface, wherepixels have brighter signal if they are in the focus plane, while thoseout of focus appear completely black. By performing a vertical scanon the surface, it is possible to collect a stack of confocal images andobtain an axial response of the focus for each pixel. Processing theaxial responses makes it possible to generate a 3D reconstruction ofthe surface.
By combining this metrology approach with an appropriate
immersion objective, the S mart sensor can measure the padasperities while the pad is still on the polisher.
Using the same immersion objective with Focus Variation
technology, groove depth and width can be quickly determined inorder to detect and monitor 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 madepossible for production environments thanks to a function within
the CMP software plug-in that determines both the width anddepth 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 inwater immersion conditions, S mart CMP is also able to characterize
pad asperity following conditioning and polishing phases. This not
only allows monitoring and thus determination of the right point
in time for conditioning, but also permits characterization of theoptimal conditioning time required to regenerate the pad surface.
All these aspects help to extend usable (and valuable) pad lifetime,
reduce the need for control wafers and additionally optimize theIn-situ metrology for pad monitoring in CMP

3D measurement of a new pad groove4process – thus also avoiding re-processing of otherwise completedwafers.
Image showing evolution of the pad asperity height during thepolishing process. After several hours of polishing a ‘glazing peak’
appears in the surface distribution. By monitoring the pad surface atintervals during the polishing process, the pad surface can be duly
glazing peakNew(a) pad
2 hr

4 hr

6 hr

interventio

conditioned (intervention) in order to revert the surface condition tothe initial state.
Image showing comparison for the pad before intervention (black)vs the pad following intervention (red). The image on the left side
was for the surface after 6 hour polishing process and the right one
is after 12 hours.

(b) 10h use
Good pad-– No Intervention– Intervention– No Intervention– Intervention

6 hour12 hour

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

Automatic analysis
To simplify surface parameter control, the analysis of the acquired
data can be automated. The operator then simply places the sensor
over the pad surface and focuses on it, and once the acquisition has
started the software will automaticallyshow the target parameter value.
Special analysis algorithms developed forthis application can automatically detectgroove width and depth to monitor
groove occlusion, while a different
algorithm is designed to analyze thecritical parameters of the asperities on thepad surface in order to detect pad glazing.

In-situ metrology for pad monitoring in CMP

5

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 SPECIFICATIONSSENSOROBJECTIVECONFOCAL

FOCUS VARIATION

Dimensions

WeightMagnification

Working
Distance (mm)Field ofView (µm)

OpticalResolution (µm)

VerticalResolution (nm)Measurement
Time (s)VerticalResolution (nm)Measurement
Time (s)26x28x37cm
10.2x11x14.6 “9 Kg
19,8 Ibs

20X(Immersion)

2.00

877x660

0.31

2020155Sensofar is a leading-edge technology company that has the highest quality
standards within the field of surface metrology.
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