Journal of Electronic Materials, Vol. 25, No. 10, 1996 Special Issue Paper

Optical Interferometry for Surface Measurements of CMP Pads

DAVID STEIN,* DALE HETHERINGTON, ~ MIKE DUGGER, * and TOM STOUT*

�9 University of New Mexico, Department of Chemical and Nuclear Engineering, Albuquerque, NM 87131

�9 Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87123

�9 WYKO Corporation, 2650 East Elvira Road, Tucson, AZ 85706

Optical interferometry was used to quantitatively characterize the surface of chemical-mechanical polishing (CMP) pads used to polish oxide films. We discuss the optical interferometry technique, including a description of the parameters necessary to compare pad samples. Flat, mesa-like structures formed on the pad during the first 5 rain polish when conditioning was not used. The data from the optical interferometer indicated that the surface topography did not change with subsequent polishing, even though the thermal oxide removal rate continued to decrease. We found conditioning roughened the pad surface. Rougher pad surfaces removed more oxide during a single 5 rain polish than comparatively smooth pad surfaces. Data indicates that conditioning increases and stabilizes pad surface roughness.

Key words: Chemical-mechanical polishing, optical interferometry, oxide films, surface morphology

INTRODUCTION

Chemical-mechanical polishing (CMP) is used in the fabrication of integrated circuits to reduce defects and to achieve adequate planarization necessitated by stringent photolithography depth of focus require- ments. 1 The fundamental physical and chemical mechanisms of CMP are not well understood; 1 under- standing is limited by a lack of quantitative informa- tion about the polishing pad surface, and the changes to the pad surface during polishing and conditioning. A quantitative description of pad surfaces formed under various conditions promotes investigations into improved conditioning techniques, pad materials, and refined models.

Techniques previously used to image and develop texture or contour maps of pad surfaces include stylus profilometry, atomic force microscopy (AFM), and scanning electron (SEM) microscopyY Stylus pro-

(Received March 30, 1996; accepted for publication May 10, 1996)

filometry is performed by dragging a stylus tip across the surface under examination, and recording its vertical displacement. The size of the tip limits the resolution capabilities of this technique; most stylus tips are approximately 0.1 to 2.5 pm.

Atomic force microscopy is performed in a similar manner, however, the tip diameter is several orders of magnitude smaller. Surface deformation due to the down force on the stylus is a concern with stylus profilometry, but not with AFM. The feature dimen- sions usually investigated by AFM techniques are much smaller than those of a polishing pad, hence useful statistical information cannot be obtained by this technique.

In contrast, instruments utilizing optical inter- ferometry can provide non-contact, high resolution surface texture mapping. The x,y range of the contour map is determined by the size of the microscope objective used, and the z range (height) is determined by mechanical movement of the sample stage. The optical interferometer surface characterization tech- nique described in this work is used to measure the

1623

1624 Stein, Hetherington, Dugger, and Stout

Fig. 1. Optical schematic of the interference microscope used in this study.

b Fig. 2. Typical contour plot of a pad surface generated by the WYKO optical interferometer. A scanning electron miroscopy image of the same sample (though different location) is shown for comparison,

effects of polishing and conditioning on chemical- mechanical polishing pads. Oxide polish rates are then compared to changes in pad texture as a function of cumulative polish time.

P A D S U R F A C E C H A R A C T E R I Z A T I O N TECHNIQUE

The optical interferometer used in this study is a white light source Mirau interferometer manufactured by WYKO instruments, shown schematically in Fig. 1. The surface heights are measured by vertically scanning the reference arm of the interferometer, and then calculating the relative modulation of the inten- sity signal as a function of vertical position. The primary difficulty with this technique is data dropout which occurs when the intensity of the light reflected from the surface is lower than the intensity threshold of the detector. This is caused by surfaces of low reflectance or steep slopes on the surface which re- flects most of the light away from the objective. Data dropout can be minimized by using brighter light sources and careful collimation of the optics.

Figure 2a is a typical contour map of a polyurethane pad surface produced by the WYKO RST. Data drop- out occurs on the steep slopes of the pores which accounts for less than 20% of the data collected and can easily be interpolated from the recorded data. Figure 2b is a top down SEM image of a polishing pad for comparison with the contour map. Comparison of the two images indicates tha t the interferometric imaging technique provides an accurate representa- tion of the pad surfaces and pore structures investi- gated in this study.

The six responses obtained from the interferometer for each sample are average roughness ( R a ) , r m s

roughness (Rq), skew, kurtosis, core roughness depth (1~), and the Swedish height (SH). The calculation methods for the first four descriptors are shown in Table I. Skew is a measure of the asymmetry of the histogram of surface heights about the mean; Gaussian distributions have skews of zero. Since skew is essen- tially the cube of the surface heights, extreme values influence skew much more than values near the mean. Kurtosis is a measure of the flatness of the profile of the histogram of surface heights. The kurto- sis of a Gaussian profile is three, profiles that are more peaked have kurtosis values greater than three, while profiles flatter than a Gaussian distribution have kurtosis values less than three.

Bearing ratio curves are another way to charac- terize and compare surfaces. The bearing ratio curve depicts the percent of planar surface area intersected by the rough surface for all heights (see Fig. 3). R k and SH are obtained from the bearing ratio curve. The core roughness depth, R k, is usually considered the working part of the material. 4 R k is calculated by finding the minimum slope of the bearing ratio curve over a 40% window, extending a line out to intersect 0 and 100% on the x axis, and calculating the height change of the line thus formed. SH is the height change between 5 and 90% on the x-axis of the bearing ratio curve and is independent of any other character- istics of the surface. Pictorial representations of R k and SH are shown in Fig. 4.

Optical Interferometry for Surface Measurements of CMP Pads 1625

E X P E R I M E N T A L

Polishing was performed with a Cybeq 3900 pol- isher which polished three wafers simultaneously. The head pressure was 7.5 psi, and the platen, carou- sel, and head speeds were 20, 15, and 5 rpm. The polish slurry was Cabot SS12, dispensed at a rate of 600 ml min -1. This flowrate creates a thin layer of slurry on the pad. Rodel IC 1000/Suba IV stacked pads were used with DF200 carrier inserts. An OnTrak scrubber was used for post-polish cleaning with dilute ammonium hydroxide. The wafers used in this s tudy were unpat terned 150 mm diameter silicon wafers with 15000A of thermally grown oxide. Film thick- nesses were measured with a Prometrix FT-530 opti- cal probe utilizing a 17 site map and 10 mm edge exclusion.

For each experiment, the pads were removed from the polisher and sectioned prior to analysis. Pad samples were cut from the center of the wafer path and were approximately two inches wide by 6 inches long. The strip was mounted on a stiff backing and secured such that the pad surface could not be dam- aged. For statistical purposes, 20 images of each sample were taken, and the data averaged for each data point shown. Samples were also obtained for SEM imaging.

R E S U L T S A N D D I S C U S S I O N

U n c o n d i t i o n e d P a d s

Unconditioned pad surfaces with 5, 15, 30, 45, and 60 min of cumulative polish time were characterized with the optical interferometer. The cumulative pol- ish times were obtained with consecutive 5 min pol- ishes. Figure 5 shows the surface characteristics and oxide thickness removed as a function of cumulative polish time. The oxide removed as a function of cumu- lative polish time shows a steady decline for all pads. The oxide removal for all cumulative polish times is lower for the pad from lot B (60 min cumulative polish time) than for the pads from lot A. The change in the

Table I. The Calculation Methods for Four Surface Descriptors are Shown

Descriptor Calculation

: ~ - IZjk I

- X Y

Rq = / XXz k q V i~ k = l j = l

Skew

Kurtosis

i 1Zz Skew- R 3 N ~-1

q

,_• z~ Kurt- R ~ N j_~

q

Note: The total number of points in the data array is N.

values of the surface characteristics from time zero (unused pad) to 5 min of cumulative polish time indicate the pad surface changes during the first 5 min polish. The data indicate the pad surface remains constant thereafter. Figure 6 shows oblique SEM images of the pad surfaces when new, after 5 min of polishing, and after 60 min of polishing. These three images show the change in the surface during the first 5 min polish and the stability of the surface between 5 and 60 min of cumulative polish time. The greatest change during the first five minutes of polishing is the formation of mesa-like structures which do not ap- pear to change in size and shape thereafter. The consistent pad topography after the first 5 min polish as indicated by the interferometer data and SEM images does not correlate with the trend in the oxide removal rate, hence the drop-offin the oxide removal rate is due to changes in other factors or surface

;I g

,/~ ' Surface Profile t

/

;!

/ J

/ /

/

Bearing Ratio ~ "

0% Bearing Ratio 100%

Fig. 3. The bearing ratio curve for a superimposed two-dimensional su trace. Construction of the bearing ratio cu we for a th ree-dimensional surface is similar.

Rk I,J

i-~ Swedish Height

0% 5% Bearing Ratio 90% 100%

Fig, 4. A typical bearing ratio curve showing the definition of core roughness depth (R,) and Swedish height.

1626 Stein, Hetherington, Dugger, and Stout

Fig. 5. The oxide thicknesses removed and pad surface characteristics as a function of cumulative polish time on the pad are shown. The pads were not conditioned and cumulative polish times were obtained using 5 min polishes. Empty symbols indicate dimensionless values (Skew and Kurtosis).

topography changes on scales not studied in this work.

Effect of Pad C o n d i t i o n i n g

The oxide removal and pad surfaces for conditioned pads were investigated for two different conditioning techniques. One conditioning technique used a nickel- bonded diamond (100 grit) encrusted edge. The sec- ond conditioning technique used first a toothed blade and then the nickel diamond edge. Figure 7 shows the tips of both conditioning tools. Figure 8 depicts the thickness of oxide removed as a function of polishing cycle and the average roughness of the pad immedi- ately following conditioning (Rat). Figure 8 also shows, for comparison, the thickness of oxide removed and the initial average roughness (Rai) of a pad used to polish without conditioning (described above). All three pads were from the same production lot. The pad surface data indicate rough pad surfaces remove more oxide in the 5 min polish cycles than smooth pad surfaces.

Pad surface roughness, on the scale examined in this study, is not the only factor that can affect oxide removal rates. Oxide removal rate also depends on the chemistry and particulates in the slurry2 The slurry in the pores of the pad will change if not replaced by fresh slurry during polishing. Condition- ing stabilizes the oxide removal rate by maintaining a consistent pad surface and possibly enhances the transport of fresh slurry into the pores of the pad.

Fig. 6. (a) An unused pad, (b) a pad with 5 min of cumulative polish time, and (c) a pad with 60 min cumulative polish time. None of the three pads were conditioned, and the cumulative polish times were obtained using 5 min polishes.

SUMMARY

We have introduced a new technique, optical inter- ferometry, for the quantitative measurement of the surface topography of CMP pads. Average roughness (Ra) , rms roughness (Rq), core roughness depth (Rk), Swedish height (SH), skew, and kurtosis were used to compare pad samples. We investigated the surface topographies of conditioned, unconditioned, new, and used pad samples. Oxide thickness removed was found to drop-off as a function of cumulative polish time on an unconditioned pad. However, the surface topography of the pad remained constant after the first 5 min polish. Hence, the drop-off in film thick- ness removal between 5 and 60 min of cumulative polish was not caused by a change in the measured pad surface. We found that conditioning roughens the pad surface. Rougher pad surfaces removed more material than smoother pad surfaces. Conditioning

Optical [nterferometry for Surface Measurements of CMP Pads 1627

b Fig. 7. The tips of the two conditioning tools are shown: (a) the toothed blade, and (b) the nickel-bonded diamond edge.

also prevented the drop-off in oxide removal as a function of cumulative polish time that was seen when conditioning techniques were not employed. We concluded that conditioning results in stabilized ox- ide removal rates by maintaining the pad topography and by possibly refreshing the slurry in the pores of the pad.

ACKNOWLEDGMENTS

The authors wish to thank WYKO instruments and

Fig. 8. The oxide thickness removed and pad surface data as a function of cumulative polish time is shown.

Scott Holswade at Sandia National Laboratories for the use of their WYKO tools, John Doyle, Phillip Monteith, Kathy McKenna, and Pat Shea for techni- cal assistance, and Linda Cecchi and Joe Cecchi for editorial assistance. This work was sponsored by DOE contract DE-AC04-94AL85000.

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