AN516 Dektak Stress Measure

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Thin Film Stress Measurement Using Dektak Stylus Profilers

By: Mike Zecchino, Tim CunninghamVeeco Instruments Inc.

deposited thin film layer, based uponthe change in curvature and materialpro p e rties of the film and substrate.

The key parameters needed to makethe stress calculation are the substrate’sradius of curvature before and afterdeposition. If the height of the substrate

is expressed as a continuous functionof distance along the substrate, y=f ( x),then the radius of curvature at anypoint may be calculated as:

where y' = dy/dx, and y'' = d 2 y / d x 2

Assuming an initially flat substrate,the stress in the film can then becalculated as:

w he re

σ = stress in the film, after deposition R p r e = substrate radius of curv ature,before deposition

R pos t = substrate radius of curva t u r e ,after deposition

= Youn g’s modulusυ = Poisson’s ratiot s = substrate thickness

t f = film thickness.

Each scan is fit with a 5th orderpolynomial by the method of leastsquares. The fit to each scan is thendifferentiated to produce the function

y’ ( x ) and again to produce y "( x ).

I n t r o d uc t ion

Film deposition and planarizationprocesses employed in themanufacture of semiconductors,MEMS, thin film filters and otherdevices, generate stress in both thesubstrate and the deposited film.Excessive film stresses can lead todeformation, cracking, delamination,sho rts and other failures that canrender a device unusable. Accurateassessment of the deformation causedby film stress is critical for developingcontrollable processes and producinghigh quality devices.

Veec o ’s Stress MeasurementAnalysis, an optional softwarepackage for Dektak® stylus profilers,calculates tensile and compressivestresses and displays the results. Thisapplication note discusses stressmeasurement methodology, thealgorithms behind the Dektak StressMeasurement Analysis, and import an tconsiderations for achieving accuratestress calculations.

Measurement Theory

The standard stress measurementtechnique is straightforward: onemeasures the curvature of the substrateprior to deposition, then measures thesubstrate along the same trace after afilm is applied. The Stress Measure-ment analysis uses the bending platemet hod1,2 to calculate stress in a

These functions are substituted intoequation 1 to calculate the radiusof curvature as a function of scanposition before and after deposition.The two radii are substituted intoequation 2 to calculate pre-depositionand post-deposition stress. The

difference between these values isthe film induced stress. Negativevalues of stress are compressive(convex surface); positive values aretensile (concave surface). The unitsof stress are dynes/cm2 (1 dyne =10-5 N e w ton ) .

Equipment List

Accurate stress measurement requires

scans across a significant portion ofthe center of the substrate (typically70% or greater), and the pre- andpost-deposition scans must representthe same portion of the substrate.While stress can be measured usingany Dektak profiler with the StressMeasurement Analysis installed, theDektak 8 Advanced DevelopmentProfiler is the recommended instrument,due to its long scanning capability (upto 50mm standard, 100mm optional),

3-point suspension stage, and optionalprogrammable theta positioning.

A larger diameter stylus (e.g.,12.5µm radius) is recommendedto allow for high speed scanningw i t ho ut sc r atc h ing.

[1] In some texts, R is approximated as r 2 / 2δ, where r is the radius of the substrate and δ is the deflectionof the substrate at its center due to film stress. This is a good approximation for average stress on shallowparabolas, such as wafers with uniform film deposition, but is not generally true for all cases, and does notcalculate variations in stress across the substrate sur f ac e.

(1 )

( 2 )



Making a Stress Measurement

1. Position the substrate on the stage’sthree alignment ball bearings(the steel balls are included inthe Stress Measurement Analysispackage). It may be necessary

to raise the stage’s position pinsslightly to accommodate the heightof the ball bearings.

T IP: Al tern ati vel y, you can placethe substrate on the bare stage,but do not apply vacuum, as thismay distort the substrate. Thethree-ball suspension used withthe Dektak 8 is more repeatableand is therefore therecommended method.

2 . Verify that the stage is level towithin 1% of the measurementrange. Auto-leveling, a feature withthe programmable stage option,simplifies the leveling process.

Now set up the system to measurepre-deposition curva t ure.

3. Choose F il e>Ne w to create anew recipe file. Select Standardrather than 3D mode.

4. Click the Scan Routines icon ,or choose Window > Sc a nR o u t i ne s to view the currentparameters. Click in the ScanParameters area to open the ScanParameters dialog box (Figure 1).

5. Set the Scan Length toapproximately 80% of thesubstrate diameter, to exclude

edge effects in the outer 10%. Thescan Duration will automatically beset to provide the fastest scan.

6. Set the scan range to the highestavailable value, to achieves u fficient resolution while also

allowing for significant stress-induced curvature.

TIP: The 1mm option for theDektak 8 is recommended forstress measurements as itaccommodates a larger degreeof substrate curv ature.

7. Choose a medium value for StylusForce. For softer film materials,reduce the force to avoid

scratching at high scan speeds.8. Under Profile choose Hills &

Valleys, then click OK to closethe window.

9. Click the Sample Positioning

icon .

10 . Move the scanner to the left edgeof the wafer (i.e., drive the stageuntil the X value is at a minimum),then move in approximately 1 ⁄ 2"from the left edge to the startlocation. Choose Edit>Enter ScanLoc a t ion to store it.

You are now ready to measurethe curvature of the substratebefore deposition.

11. Click the Scan button tomeasure the bare substrate.

12. The Dektak software’s data levelingalgorithm improves measurementa ccurac y. To invoke it, click anddrag the R (Reference) and M(Measurement) cursors toapproximately 500 µm from eitherend of the scan. ChooseEdit>Enter Software Leveling.Click the Level the Trace icon .The results will appear similar toFigure 2.

1 3 .Choose F i l e >Sav e to store thepre-deposition data.

TI P : It is a good idea to save your measurement automationat this point as well. Open theAutomation Program screen ,then choose File>Save As. Withthe data stored you will be ableto complete the stress measure-me nt, even if the system isdisturbed in some way during

the deposition step.1 4 . Choose Sample Positioning ,

then click the Unload Stage button

to remove the wafer for filmdeposition. The stylus tower willautomatically be raised.

1 5 . Following deposition, repositionthe coated substrate on the stage,paying careful attention to notch/flat alignment.

Figure 1. Scan Parameters dialog box.

Figure 2. Pre-deposition wafer scanafter software leveling.



16. Click the Scan button to scanthe coated substrate. The stylustower will automatically belowered and the scan will begin.

17. Choose Fi l e > Sav e to store yourpost-deposition scan data.

You now have all the data you need torun the Stress Measurement Analysis.

18. With the post-deposition scandata on screen, chooseAnalysis>Compute Stress. Thiswill open the Stress ParametersDialog Box (Figure 3).

1 9 . To the right of the Elasticity box,click Select. You will be presentedwith a list of standard substrate

materials (Figure 4). Choose thesubstrate material to obtain itselasticity constant, then click OK.If the actual value for yoursubstrate is different than thatgiven value, you can change it inthe Elasticity field.

N OTE: The measured values ofYou ng ’s Modulus and Poisson’sRatio vary considerably fromsource to source depending on

measurement method. For Siliconin the <100> and <111>orientations, Dektak stresscalculation software uses thevalues measured by Brantley3.

20. Enter the substrate and filmthicknesses in microns.

21. At the bottom of the dialog, clickSelect to locate the pre-depositionscan data. Click OK to calculatethe film stress and display the

results (Figure 5). The results canbe printed as well. The results arealso displayed as a plot (Figure6). Downward curvature in theplot indicates compressive stress;upward curvature indicatestensile stress.

Figure 3. Scan Parameters dialog box.

Figure 4. Choosethe substrate materialto obtain its elasticityco ns tan t .

Figure 5. Stress Results Summary Window.

Figure 6. Stress Measurement results. The orange trace indicates that both tensileand compressive stresses are present following deposition.



Solutions for a nanoscale world.

Veeco Instruments Inc.2650 E. Elvira Road

Tucson, AZ 85706 USAFind out more at www.veeco.com

520-741-1044 · 1-888-24-VEECO

AN516-1-0204

© 2004 Veeco Instruments Inc. All rights reserved.

Dektak is a registered trademark of Veeco Instruments Inc.

References

1. G. Stoney, Proc. R. Soc. London Ser. A, Vol. 82, (1909) p.172

2. R.A. Jaccodine, W.A. Schlegel, J. Appl. Phys., Vol. 37 (1966) p.2429-34

3. W.A. Brantley, J. Appl. Phys., Vol. 44 (1973) p.5344-5

4. M.E. Thomas, M.P. Hartnett, J.G. McKay, National Semiconductor Corp.,The Use of Surface Profilometers for the Measurement of Wafer Curvature,Vacuum Science and Technology, Vol. 6, 1988

Measurement Ti p s

1 . The Stress Measurement analysisis designed to characterizeu np a t te rned wafers. Surfac efeatures will skew the curve fitsand may lead to inaccuratestress measurement.

2 . Use appropriate fixtures to ensurethat the pre-deposition and post-deposition scans overlapp r e cisel y. Bear in mind that s tressis calculated from the change insubstrate curvature, not from theabsolute curvature; therefore,slight offsets in substrate positioncan produce large errors in thestress results.

3. P e rf o rm two stress measurementsat 90 degree angles to achievea more complete image of thesubstrate distort ion .

4 . If the substrate has been heatedor cooled prior to stress measure-ment, let it reach therm a lequilibrium before scanning.

5 . Near the edges of a scan, thefirst and second derivatives of the

curve fits will deviate becausethey are unconstrained by actualscan data. Place the R and Mcursors at least 10% of the scanlength away from each edge toexclude the ends of the scan.The stress calculation will still beplotted to the edges, but the

s u m m ary data in the Stress Resultswindow will reflect only the areabetween the cursors.

6. It is important to make a pre-deposition scan on the actualsubstrate, rather than substituting

a measurement of a generic, flats u rface such as an optical flat orstraight line. Unless the actualsubstrate is exceptionally flat,such a substitution will lead toan inaccurate assessment of thefilm stress.

7. Both pre- and post-depositionscans must have the same numberof data points. Do not abort eitherscan before completion.

8 . Based on user experience withproduction silicon wafers4, stressresolution down to the order of1 x 108 d y ne/ c m2 is practical.Very low film-induced curv a tu r emay lead to stress calculationswhich are below the practicalresolution of the profiler.

Conclusion

Accurate stress measurement helpsprocess developers avoid non-planarity effects to ensure uniformdeposition and high partp e rformance. for samples up to200mm in diameter. The StressMeasurement Analysis option forDektak stylus profilers gives you theflexibility to accurately characterizefilm stress, as well as roughness andsteps, for comprehensive film analysison a single platform .

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AN516 Dektak Stress Measure