Metrology For Characterization Of Wafer Thickness .
Metrology for Characterization of Wafer Thickness Uniformity During 3D-IC ProcessingAuthors: Tom Dunn, Chris Lee, Mark Tronolone, Aric ShoreyCorning IncorporatedCorning, New York 14831ShoreyAB@corning.comAbstractThere is a constant desire to increase substrate size in orderto improve cost effectiveness of semiconductor processes.As the wafer diameter has increased from 2” to 12”, thethickness has remained largely the same, resulting in a waferform factor with inherently low stiffness. Gravity induceddeformation becomes important when using traditionalmetrology tools and mounting strategies to characterizea wafer with such low stiffness. While there are strategiesused to try to reduce the effects of deformation, gravitationalsag provides a large source of error in measurements.Furthermore, glass is becoming an important material forsubstrates in semiconductor applications and metrology toolsdeveloped for use for characterizing silicon are inherentlyless suitable for glass. Using a novel mounting strategy anda measurement technique based on optical interferenceprovides an opportunity to improve on the methodologiesutilized to characterize wafer flatness (warp, bow) and totalthickness variation (TTV). Not only can the accuracy of themeasurement be improved, using an interference basedtechnique allows for full wafer characterization with spatialresolution better than 1 mm, providing substantially morecomplete wafer characterization.IntroductionHistorically, use of glass wafers in the semiconductorindustry has been primarily for MEMs and CMOS imagesensor applications. These applications typically had loosespecifications for TTV and warp. Using glass as a carrier waferfor precision thinning of silicon in 3D-IC applications requiresthat the thickness uniformity and warp are tightly controlledsince non-uniformities in the carrier directly impact theaccuracy of the silicon TTV.Another challenge is given by the fact, that over the pastseveral years, wafer diameters have increased dramatically;resulting in the requirement to accurately characterizesextremely high aspect ratio wafers (300 mm diameter andthickness 1 mm). High aspect ratio parts have inherently lowstiffness and characterizing the flatness of such a componentis extremely challenging due to gravity effects. Conventionalmounting methods, e.g. three/four-point mounts, are lesssuitable for flatness characterization of such high aspectratio parts due to a great deal of deflection of gravity andsensitivity to how the wafer is placed on the mount. Thisleads to erroneous results when trying to characterize warpand bow.Corning Tropel has developed a novel distance measuringinterferometer based on a frequency stepping laser that iswell-suited to characterize the flatness and TTV of glasswafers. In fact, several commercial interferometers capableof characterizing the flatness, thickness, and TTV of 300 mmdiameter glass wafers have been installed. In addition to novelmounting strategies that substantially avoid errors given byhistorical techniques; this metrology tool has extremely highaccuracy and a tight pixel density. A 300 mm diameter waferwould have millimeter-level lateral resolution as comparedto profile based resolution given by existing techniques. Thisgreater data density provides extremely valuable informationto the quality of the wafers.We will compare and contrast different metrology techniquesand their relative attributes and discuss additionaldevelopments in using this technology. The significantadvantages provided by this approach for precisioncharacterization of wafers and wafer stacks will also beprovided.BackgroundIn the beginning the semiconductor industry was just anemerging market. Lithography as it is done today was beyondthe imagination of even the people at the leading edge ofthis new technology revolution. Wafers were small, first 1inch then 2 inches in diameter. The industry was lookingfor consistent characterization of these small thin wafers toestablish standard quality.What was it they wanted to characterize? They were lookingfor a measure of the degree of convex/concave shape in thewafer, and an overall wafer flatness measurement.With the wafer sizes of the time, it was desirable to supportthe wafer in a simple manner that was easily reproducible,so the three-point mount was perfect. It is a kinematicsupport, so any three-point support should result in the samedeflections. A small misalignment of the part would result ina relatively small reproducibility error.However, measuring the concave/convex magnitude becomestrivial in this fixture. You can simply measure the center pointand compare the measurement to the measurement of anoptical flat supported by the same three points. This thenbecomes a measure of the sag of the wafer. There is a smallamount of gravitational influence, but this should remainrelatively constant from wafer to wafer for the same nominalgeometry. The beauty of this measurement is that you canmeasure bow with a single point probe on a fixed jig. Filmstress could be correlated directly to the magnitude of thechange in this bow measurement (e.g. see Stoney’s equation)after the application of the film.1
Measuring warp still requires full surface information, butthe three-point support allows the measurement to be madedirectly without the complication of calculating the leastsquares plane, making it convenient back when these typesof analysis were limiting factors.Over time the diameter of the wafer grew, but the thicknessdid not increase proportionally, 3 inch, then 4 inch, 5 inch for awhile, then 6 inch, 8 inch, and now 300 mm (12 inch) with 450mm (18 inch) just over the horizon. This seems like no majorconcern, but all those small errors associated with minoralignment errors start to become very significant relative tothe target values of bow/warp.Another challenge arises from variation from how the waferis mounted on the metrology tool. Often times a threepoint mount is used with characterizing a wafer, but fourpoint mounts, ring supports and others are also utilized.Deformation from gravity will significantly differ in shapeand magnitude depending on how the wafer is held duringcharacterization. Figure 1 shows the shape of a theoreticallyperfect (TTV and flatness 0 μm) wafer if it is held at theperimeter by a three-point (1a), four-point (1b), or ringsupport (1c). The magnitude of the total deflection (sag) isalso strongly related to the mounting strategy. As indicatedin Figure 1, the calculated deflection through finite elementanalysis (FEA) modeling of a 300 mm diameter, 0.7 mm thicksupported at the perimeter by three-point, four-point andring support is 206 um, 160 um and 130 um respectively.(1a) Three-pointmount at perimeterSag: 206 μm(1b) Four-point mountat perimeterSag: 160 μm(1c) Ring support atperimeterSag: 130 μmFigure 1. FEA showing shape of deformation with differentsupport levelsThe effect of how the wafer is supported on the total sag ofthe wafer was discussed above. There can also be substantialchanges in variation by small changes in the wafer properties,mounted under the same conditions. Let’s consider a fewsimple cases:Wafer diameter: 300 /- 1 mmWafer material: SiliconDensity: 2.33 g/cm3,Elastic modulus: 141 GPaPoisson’s ratio: 0.22Wafer thickness: 0.7 mm /- 0.01 mmThree-point support radius: 147 mmGlass Material: Corning SGW3Density: 2.38 g/cm3,Elastic modulus: 74 GPa,Poisson’s ratio: 0.23Wafer thickness: 0.7 mm /- 0.01 mmThree-point support radius: 147 mmThe first thing to note is that for these material properties,the magnitude of the gravitational sag from a three-pointsupport at -3 mm from the edge is 206 microns, which iscertainly not negligible. Compare this to the results for a0.4 mm thick, 50 mm diameter Si wafer, which has a sag ofjust over 0.35 microns. This, you can argue, is negligible, thevariation from loading is almost certainly negligible, and thevariation from different wafer thickness is also negligible.If you consider our 300 mm wafer case, simply varying thethickness of the wafer from 0.69 mm to 0.71 mm changes thegravitational influence by over 12 microns.Clearly for getting a meaningful measurement with a threepoint support requires compensating for the influence ofgravity. However as we can see from the sensitivity to thewafer thickness, the compensation is highly sensitive tovariations from wafer to wafer. Even with constant wafergeometry and properties, measuring a wafer with this kindof magnitude from gravity becomes unnecessarily complex,and incredibly sensitive to load orientation. For waferswith relatively loose tolerances, 10 μm TTV and 200 μmwarp for example, this may appropriate. However, gravitycompensation is a questionable strategy to obtain accuratemeasurements for 300 mm wafers with 2 μm or 3 μm of TTVand 40 μm or 50 μm of warp. Efforts underway to increasethe diameter of the semiconductor wafers will exacerbatethe issue. For the purpose of illustration, consider the same0.7 mm thick wafer, with a 450 mm diameter. This will sagmore than 1000 um.Different materials such as glass, typically have stiffnesslower than silicon and the influence of gravity becomes evengreater. For example, a glass wafer with the same geometryas our 300 mm silicon wafer will sag by 404 microns insteadof the 206 microns described previously. Table 1 summarizesthe sag as a function of wafer diameter, thickness andmaterial. With larger and thinner wafers, a three-pointsupport is likely to introduce as much uncertainty in themeasurement as the magnitudes of the real wafer flatness.Using other non-kinematic support methods will not allowfor accurate compensation n4500.702221060SGW33000.70147404Table 1. Summary of wafer sag variability with waferdimension/material2
Ultimately, what needs to be considered is what attribute isbeing characterized, and what is the best approach. For nonsilicon substrates, many users have changed from a threepoint supported warp measurement to what is known asSori, which measures the wafer as it would sit in its free stateon a flat plate. Naturally the wafer will still bend under theinfluence of gravity, but this will represent the same conditionthis wafer will “see” during its useful life. Also this does notrequire a compensation for gravity, as the gravity is a part ofthe measurement condition. This is especially beneficial forvery thin very large diameter wafers as they no longer haverigidity, so errors in the gravitation correction can be biggerthan the real “flatness”. If the gravity compensation is 90%correct, for these cases that would still represent a 30-40micron error, which in most cases is probably larger than thefree state flatness, and certainly larger than the Sori.Some studies around Sori measurement have also highlightedthe errors introduced when using a three-point mount formeasuring silicon wafers. In one case, supports positionedincorrectly by 2 mm caused 10 um of error in the Sorimeasurement.3Challenges discussed above are highlighted in industrystandards that discuss methods for measuring warp and TTVfor silicon wafers. 4,5 Among the limitations listed are: If there are substantial differences in diaeter, thickness,fiducials, or crystal orientation from that used forgravitation compensation procedure, the results may beincorrect. Different methods of implementing gravitionalcompensation give different results. Different geometric configurations of wafer holding(e.g. three-point, four-point, ring support, etc.) will yielddifferent results. The quantity of data points and their spacing may affectthe measurement results. Results obtained with differentdata point spacing using the same tests may be different. TTV and warp are determined using partial scan patterns;thus, the entire surface is not sampled and use ofanother scan pattern may not yield the same results. Certain test methods do not completely separate TTVfrom warp. Running probes off the test specimen during the scansequence gives false readings.actual direct measured data points. Contrast this to currentmethods which are frequently hundreds or maybe a fewthousand points, with extensive interpolation, which meansmuch of the wafer remains uncharacterized. For glass wafers(transparent at operating wavelength) the FlatMaster MSP300 enables simultaneous measurement of flatness, thickness,and TTV. The system provides the ability to characterizeup to 1 mm of bow with micron level accuracy. Thicknessand TTV accuracy are 1 um and 0.1 um respectively. Thisproduction worthy system has several units already installed.This interferometer design is based on a novel frequencystepping laser that is tunable over 30 nm. Conventionaltunable lasers provide continuous tuning over a range ofwavelengths without any mode transitions. An interferometricimage is collected at consecutive laser mode frequenciesmaking it very easy to perform Fourier transforms. Themodulation frequency of the interference on each pixel isdirectly proportional to the optical path difference betweenthe reference and test arms of the interferometer as wellas the laser mode spacing. The inherent stability of thefrequency stepping laser results in a very accurate conversionfrom the modulation frequency of the pixel to its opticalpath difference (OPD). A Fourier transform is performedon each pixel to determine the height difference betweenthe reference and measurement arms independent of itsneighboring pixels. The laser mode spacing combined withconventional phase measuring interferomer (PMI) techniquesgive the ability to achieve sub-nanometer resolution. Thistechnique can be applied to both rough and smooth partsmaking it possible to perform metrology on 300 mm glassand silicon wafers to measure flatness, thickness, and TTV.A New Method for Wafer CharacterizationA new interferometric measurement technique has beendeveloped to overcome the limitations described above.1 TheFlatMaster MSP-300 (Multi-Surface Profiler) System (seeFigure 2) is based on a new frequency scanning technology.This system has a field of view of 305 mm and measuresabsolute height, flatness and parallelism of multiple surfaces.It is well-suited to quickly ( 1 minute total measurement time)characterize wafer flatness (warp, bow) of silicon and glasswafers with vertical accuracy of 1 um. The system utilizes a2k x 2k camera, which gives sub-millimeter lateral resolutionin wafer characterization. Each pixel of the camera representsa point of direct measurement on the wafer – means that on Figure 2. Picture of the FlatMaster MSP-300 interferometera 300 mm diameter wafers there is on the order of 3 million3
ResultsA series of tests were done to evaluate the ability of the FlatMaster MSP-300 to characterize glass wafers. The first testwas to evaluate the repeatability of the wire support mountrelative to the three-point mount. For this test the threepoint mount was placed at a location at 60% of the waferradius to minimize the total deflection (assuming minimalvariation at minimum sag). Figure 6a shows a glass wafer being measured on the FlatMaster MSP-300 using the threepoint mount support and Figure 6b shows a wafer mountedon the wire support.Figure 3. Interferogram of a 300 mm glass wafer.The interferogram shows interference fringes for bothflatness and TTV.Figure 3 shows the interference fringes from a glass waferbeing measured on the FlatMaster MSP-300. Note that theinterferogram consists of fringes due to both variations inflatness and TTV. The software used in the FlatMaster MSP300 can separate these fringes and allow for simultaneouscharacterization of flatness (bow, warp) and TTV. Figure 4shows typical output maps of the TTV (1.4 /- 0.2 um ) andflatness (1.4 /- 0.2 um) from a glass wafer (error represents 1standard deviation). The high data density gives substantiallymore data fidelity as compared with scan based techniquestypically used today.A test was then done where the same wafer was measured10 times using both mounting methods. The bow and warpfor each mounting method is given in Figure 7. It is clearlyseen that not only does the wire support substantially reduce warp and bow induced by gravity, the variation inmeasured warp and bow is much higher for the three-pointmount. Given that the measurement system was identical ineach case, this increased variation can be attributed to nonrepeatability from the mounting strategy. Figure 8 showsthe thickness and TTV results from this test. Note first thatthe wire support method gives thickness repeatability better than 0.03 um and TTV repeatability 0.003 um. The relatively large sag given by the three-point mounting methoddegrades repeatability of thickness and TTV measurements,but it is still quite good at 0.1 um and 0.01 um respectively.The method of support used in the FlatMaster MSP-300 isa series of very thin wires as shown schematically in Figure5. Notice the faint vertical lines seen in the interferogram inFigure 3. This level of support prevents the large gravitationaldeflections given by more traditional techniques discussedabove. Finite element models show that the same wafer (300mm diameter, 0.7 mm thick) that gives 100’s of um of deflectionusing three-point or four-point mounts discussed above, wouldhave 1 um of deflection due to gravity effects. This meansthat flatness measurements would be insignificantly affectedby gravitational effects.(a) Average TTV 1.4 /- 0.2 umWaferHarpWireStageOrientation MarkFigure 5. Schematic Showing wire support methodology tosupport wafers in the FlatMaster MSP-300(b) Average Flatness – 17 /- 5 umThree-point MountFigure 4. Data maps showing (a) TTV ( 1.4 um) and(b) flatness ( 17 um) of a glass wafer with sub-millimeterlateral resolution.Figure 6a. Glass wafer on FlatMaster MSP-300 using athree-point mount4
Figure 6b. Glass wafer on FlatMaster MSP-300on wire supportCommercially available FlatMaster MSP-300 tools operate atvisible wavelengths. It is possible to outfit the system usinglight sources and cameras that operate in the infrared, wheresilicon is transparent. A FlatMaster MSP-300 system wasmodified to enable characterization of a silicon wafer that hadbeen mounted to a glass carrier and thinned. Figure 9 showsthe interferogram of this wafer and Figure 10 shows the resulting TTV map. The error in the TTV map after thinning was 0.75um. The high lateral resolution of the camera gives high density data that even allows you to see the grinding marks in thesilicon, giving the possibility for substantial process knowledgeto be gathered.Figure 9. Interferogram of a thinned silicon wafercharacterized using a FlatMaster MSP-300in the infrared regime.8070Flatness (μm)605040Warp - 3-PtBow - 3-PtWarp - WireBow - Wire3020Figure 10. Data from characterizing a thinned silicon waferusing showing 0.75 um deviation, and grinding marksare clearly seen.100-1012345678910IterationFigure 7. Resulting warp and bow for wire support andthree-point support methods.1.25700.81.23700.61.21Thickness (μm)()701700.41.19Thickness - 3-PtThickness - WireTTV - 3 PtTTV - Wire700.27001.171.15123456Iteration78910Figure 8. Resulting average thickness and TTV as measured bywire support and three-point support methods.ConclusionsAs the diameter of semiconductor wafers continues toincrease, gravity induced deformation becomes important.Traditionally used metrology tools and mounting strategiesto characterize a wafer with such low stiffness can leadto large source of error in measurements. Furthermore,glass is becoming an important material for substratesin semiconductor applications, making metrology toolsdeveloped for use for characterizing silicon less suitable forthe overall need to characterize semiconductor wafers tohigh precision. Gravity compensation of 300 mm diameterwafers is documented in this paper unsuitable for today’stightest wafer specs of 1.
industry has been primarily for MEMs and CMOS image sensor applications. These applications typically had loose . developments in using this technology. The signifi cant . optical fl at supported by the same three points. This then becomes a measure of the sag of the wafer. There is a small
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