B I O G R A P H YAngelo Gaitas is the pres-ident and CEO of PicoCalInc. and a research associ-ate in the Electrical Engi-neering and ComputerScience Department atthe University of Michi-gan, Ann Arbor. He received an MBA fromthe University of Wisconsin, Madison, andan MS in solid-state physics from the Uni-versity of London. His research interestsspan a variety of scanning probe microscopetechniques for manufacturing applications.

A B S T R A C TThis article describes the results obtainedwith a surface micromachined probe forscanning thermal microscopy. The probeuses polyimide as the structural materialand an embedded thin-film metal resistor asthe sensing element. The typical dimensionsof a probe are 250 µm in length, 50 µm inwidth, and 3-10 µm in thickness. The probehas measured spring constant less than 0.1 N m-1, and about 40 V nominal resis-tance. It offers a tip diameter of 100 nm. Theprobe was used to map the spatial variationin thermal conductance of various test sam-ples. Surface and subsurface characteristicswere observed.

K E Y W O R D Sscanning probe microscopy, atomic forcemicroscopy, scanning thermal microscopy,microthermal analysis, polymers, failureanalysis, nanoscience, nanotechnology

A C K N O W L E D G E M E N T SThe author would like to thank Prof. YogeshGianchandani, Dr Song Xu and Dr ShamusMcNamara for their valuable contributionsto this paper, and Sematech for providingsamples.

A U T H O R D E TA I L SAngelo Gaitas, PicoCal Inc., PO Box 131490, Ann Arbor, MI 48113-1490, USATel: +1 734 913 2608Email: angelo@picocal.com

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I N T R O D U C T I O NThermal measurements at the nanometerscale are of both scientific and industrial inter-est. Over the past decade, scanningmicroscopy using thermally sensitive probeshas been used in a variety of applications. Forinstance, scanning thermal microscopy (SThM)has been used for ultralarge-scale integration(ULSI) lithography research and cellular diag-nostics in biochemistry [1-3], detecting para-meters such as phase changes in polymerblends [4], Joule heating [5], for measuringmaterial variations in semiconductor devices[6], and subsurface imaging of metal particles[4]. Furthermore, SThM has been used to per-form near-field photothermal microspec-troscopy [8]. Finally, it has been used for datastorage and many other applications [9-11].

Various thermal probes have been devel-oped since the invention of scanning thermalmicroscopy by Williams and Wickramasinghein 1986 [12]. These probes are generally madefrom thin dielectric films on a silicon substrateand use a metal or semiconductor filmbolometer to sense the tip temperature.Other approaches, using more involved micro-machining methods, have also been reported[13]. In a bolometer probe, such as the oneused in this study, the resistor is used as a localheater and the fractional change in probe resis-tance is used to detect the temperature and/orthe thermal conductance of the sample [14].

Thermal probes are used to map the spatialvariation in thermal conductance of varioustest samples whose subsurface variations arenot detectable topographically. This articlepresents a preliminary study of subsurfaceimaging on copper wires using a polyimidethermal probe. The ultimate goal of this effortis to address the semiconductor industry’s chal-

lenge to develop non-destructive in-line view-ing of copper voids. The use of SThM holds sig-nificant promise to detect defects such as voidsin copper lines in advanced complementarymetal oxide semiconductor (CMOS) processes.Since copper interconnects are common inadvanced CMOS devices, it is vital for the semi-conductor industry to obtain timely informa-tion about the quality of the copper electro-plating process and related steps.

The use of non-electrical inspection meth-ods for copper electroplating has several limi-tations. Because copper is opaque, opticalinspection methods are difficult. In addition,since most of these failures occur on the inte-rior of the copper trace, their detection is alsodifficult with topographic measurementmethods using an atomic force microscope(AFM) or scanning electron microscope (SEM).While electrical methods are accurate, theyrequire at least two points of contact and spe-cial geometries that permit access to both endsof the trace to be measured; this access is usu-ally not available without special test struc-tures. Another option, thermal measurementmethods, can potentially overcome the prob-lems posed by optical, AFM, SEM or electricaltest methods. Such techniques measure theinterior of the copper trace, enabling the non-destructive localized detection of voids in thecopper. Thermal measurements require only asingle point of contact and permit inspectionof all the copper traces, regardless of geome-try. Vias may be inspected because the barrierlayer or remaining dielectric material have amuch higher thermal resistance compared tocopper.

Scanning thermal probes fabricated by six-to seven-mask surface micromachiningprocesses using polyimide as the cantilever

Polyimide Probes for Contact-Mode SPMSubsurface Thermal Imaging Applications Angelo Gaitas, PicoCal Inc. and University of Michigan, Ann Arbor, USA

Figure 1: (A) Schematic of the probe dieincluding the probe cantileverand tip. Reprinted from [1]with permission. (B) Scanning electron micro-scope image of the probe.Reprinted from [1] with per-mission. (C) SEM image of an eight-probe array. Reprinted from[20] with permission. (D) SEM image of a tip.

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material have been previously reported[1,2,14,15]. These probes have been used fortemperature mapping and subsurface imag-ing [15], for microcalorimetry applications tomeasure the glass transition temperature in aphotoresist [1,2,14], and for maskless sub-micrometer thermochemical patterning ofphotoresists [16]. Hendatro et al. [17] used theprobes for the detection of hot-spots in inte-grated circuits (IC) revealing that the highestamplitude of thermal waves generated by anoperating nMOSFET (n-type metal-oxide semi-conductor field-effect transistor) is located at aregion close to the drain area. Basu et al. [19]have been using the probes for microfluidics-related work, namely for high-speed liquidpumping, mixing and particle entrapment inthin layers of oil and water. The probes havebeen arrayed into a multi-probe system forhigher throughput large area scanning [20].An eight-probe array, such as the one in Figure1C, has been used to produce composite ther-mal images of various commercial ICs. Finally,the probes were used for high-speed contactmode topography achieving rates of 48 Hz(1.47 mm s-1) and for lateral force scans, sug-gesting that polyimide is a more suitable struc-tural material for cantilevers used in lateralforce measurements [21].

M AT E R I A L S A N D M E T H O D S

Structure and fabricationThe structure of the polyimide probe is shownin Figure 1. The probe tip diameter used wasless than 100 nm but the probe tip can be fur-ther reduced to below 50 nm with oxide sharp-ening. The probe had a topographical resolu-tion of <1 nm and a spring constant of <0.1 Nm-1. The tip height was 8 µm, and the can-tilever’s dimensions were 250 µm 3 50 µm 33 µm. The cantilever material was polyimidewith an embedded thin wire of Cr/Au, whichalso served as a sensing element. The tip wasalso made of Cr/Au. The probe had spatial res-olution of less than 100 nm. Thermal conduc-tance changes of the order of 3 pW K-1 havebeen measured. A comparison with a Wollas-ton wire thermal probe is presented in Table 1.

The probes were microfabricated in a seven-masking step sequence. Initially, a mold for thetip was created by anisotropic wet etching ona Si(100) substrate. Then a sacrificial layer wasdeposited and patterned, followed by thelower polyimide and the metals. Later, the sec-ond polyimide layer was deposited and pat-terned, followed by a gold layer, which wasused for thermocompression bonding andserved as a mirror. Finally, the probe wasreleased, flipped over, and held in place by athermocompression bond.

Interface circuit and setupThere are many methods by which a thermalprobe may be utilized. It can be operated in apassive manner whereby the tip temperatureattains the localized sample temperature. Inorder to map the thermal conductance of sam-ples, the probe is typically operated at an ele-vated temperature. The varying heat loss ismonitored by its effect on the tip through the

sample to the chuck below, which is held atroom temperature. The simplest interface cir-cuit operates in approximately constant powermode where an open-loop interface circuit isused to gauge the probe resistance change(thermal conductance change), which can becalculated from the output voltage change.The interface circuit includes a Wheatstonebridge, gain stages, and filters to reduce noise.The output voltage is plotted for the thermalimage. In the case of thermal conductancecontrast mapping, the change in probe resis-tance is proportional to the change in outputvoltage. The supplied power change is equalto the conductive heat loss between the tipand sample, which is proportional to thechange in the thermal conductance of thesample. Thus the change in output voltagerepresents the thermal conductance contrastof the sample [1].

Alternatively, the scanning thermal probemay be operated at a constant tip tempera-ture and the power required to keep the tem-perature constant is measured (closed-loopmode – feedback required). This method per-mits contrast imaging and thermal conductiv-ity measurements to be performed. When theWheatstone bridge comes out of balance, aninstrumentation amplifier amplifies thechange in voltage. Subsequently, the changein voltage is fed into a proportional-integral(PI) controller that provides a compensationcurrent to keep the bridge balanced. The aver-

age probe temperature increases or decreaseswith the compensation power, so that theprobe resistance is adjusted by a compensa-tion current through the PI controller until thechange in voltage is zero. By increasing thetemperature control resistance, the proberesistance is also increased.

An AC thermal dither may be applied to theprobe to improve thermal resolution or to per-form thermal capacitance measurements. Thethermal resolution is improved by filtering thesignal through a bandpass filter to reduce thenoise level. Since the thermal wave generatedis an evanescent wave, the AC thermal dithermay be used to control the effective probedepth. Higher frequencies of operation reducethe effective probe depth.

AFM systems for thermal probesThe probes may be operated with an AFM sys-tem. The thermal information from the probeswas fed to a circuit module such as the onedescribed above, which in return interfacedwith an AFM controller (Figure 2A). In thesemeasurments the probe was operated in con-tact mode by scanning a thermal probe tipacross the sample and making measurementsat discrete points. Thermal probes operated incontact mode show improved performance.By contrast, operation in a tapping or non-contact mode has several disadvantages. First,the temperature sensitivity of the probe iscompromised because of the large thermal

Figure 2: Comparison of original AFMimaging mode (A) and a simpler system for thermalmeasurements in which Z-axisactuation is eliminated (B).

Figure 3: AFM images taken without Z-direc-tion feedback. (A) Scans of a developed UV6 pho-toresist. The photoresist pattern is350-nm thick and 500-nm wide [1].(B) Map of thermal conductance of adeveloped PMMA photoresist on a 4-inch silicon wafer. The photoresist pat-tern was 240-nm thick and 200-nmwide with a pitch of 400 nm.Reprinted from [22] with permission.

A B

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resistance of the air gap. Second, spatial reso-lution is reduced because the effective sensingarea is enlarged as the distance between thesensor and the sample increases. Third, highstiffness in the probe is required which maycause damage to soft samples. The use of poly-imide probes eliminates these problems.

Moving on from the original approach, asimpler system has been devised (Figure 2B),which obtains only thermal information anddoes not require Z-axis feedback. The highcompliance of the probe allows scans of sam-ples with large topographic variations. An X-Ystage controls the position of the probe whilean interface circuit is used between the dataacquisition computer and the thermal probe.A simple construction is enabled by eliminat-ing Z-axis actuation and the hardware that isneeded for it, such as photodetectors, lasers,and other electronics. An additional advan-tage is that the probes can be arrayed for high-throughput large-area scanning (Figure 1C).

R E S U LT S A N D D I S C U S S I O N

SPM scansFigure 3 contains scan images obtained with-out contact force feedback control using thesystem described above and depicted in Figure2B. Scans of a developed Shipley UV-6 deepultraviolet photoresist made without Z-direc-tion feedback are shown in Figure 3A. Thephotoresist pattern was 350 nm thick and 500nm wide. Comparing the line profiles with andwithout Z-direction feedback, the with-feed-back operation provided higher signal-to-noise ratios. The fluctuation of the tip-samplecontact area was larger without feedback. Fig-ure 3B shows a thermal conductance map ofdeveloped polymethylmethacrylate (PMMA)on a four-inch silicon wafer. The photoresistpattern was 240-nm thick and 200-nm widewith a pitch of 400 nm. The scan resultsshowed that the thermal probe can provide aspatial resolution better than 200 nm withoutcontact-force feedback control [22].

Subsurface imaging capability is very usefulfor measuring semiconductor devices wheremultiple layers are present and the final IC iscoated with a passivation layer. Thermalimages showing metal lines through a passi-vation layer were obtained. Results demon-strating the subsurface imaging capability ofthe thermal probe are shown in Figure 4. Asample containing 50-nm thick chromium lineson a glass substrate was coated with a 5-µmthick planarized photoresist, which had a ther-mal resistivity of 0.193 W m-1 K-1 (Figure 4A). Atopographical image of the sample showedthat the photoresist was uniform and theunderlying Cr layers were not detected (Figure4B). The thermal image, on the other hand,clearly detected the underlying Cr layers. Thevariation in thermal resistance amounted to a1% change in 1.0 3 1010 K W-1 and the signal-to-noise ratio was in excess of 15, as shown inFigure 4C [15].

Figure 5 illustrates another example of sub-surface mapping. The sample contained 90-nm wide Cu lines covered with 250 to 300-

nm wide and 125-nm thick natural oxide. AnSEM picture of the trench before Cu deposi-tion is shown in Figure 4A. An SEM picture ofa Cu line is shown in Figure 5B. An AFM scan ofthe Cu lines covered with a thick layer of oxideis shown in Figure 5C. Figures 5D and 5E illus-trate overlays of thermal scans superimposedon topographical scans and show <300 nm dis-continuities in thermal conductance mapsoccurring under the natural oxide. These dis-continuities were not visible topographically.The images revealed subsurface informationabout the Cu lines, which potentially may berelated to Cu voids. The current through theprobe was 12 mA and the nominal probe resis-tance was 26 V. The area scanned was 6 3 6µm2 and the scan rate was set at 1 Hz for Fig.5D and 1.4 Hz for Fig. 5E.

SimulationsNumerical simulations were performed inorder to demonstrate the feasibility of detect-ing voids in copper lines and to enhanceunderstanding of the quality and detectabilityof the thermal conductance signal. Thermalscans over copper lines having various types ofvoids with different sizes and locations weresimulated using the Femlab 3 MultiphysicsModeling package by Comsol [23]. Each simu-lation yielded maps of the change in thermal

conductance as an area the size of the probe-tip heated the surface of the simulated copperlines.

The simulated structure was based on Intel’s130-nm process [24]. The structure consistedof a 400-nm thick lower layer of field oxide,the bottom of which was held at 0oC. The toplayer was a dielectric, 280-nm thick, 1-µmwide, and 1-µm long, with a copper feature,150-nm wide, 150-nm long, and 280-nm thick,located at the center of the dielectric. A voidwas simulated in the copper layer and its loca-tion and size were varied. A cylindrical thermalprobe with 50-nm diameter resided on top ofthe copper and the probe temperature washeld at 100oC at the point of contact with thecopper.

Simulations of features with and withoutvoids were performed and the heat flux out ofthe thermal probe was calculated. The num-ber of bits of resolution in the sensed signalrequired in order to detect a particular voidwas determined from the difference in ther-mal resistance with and without a void for aparticular depth. The simulations confirmedthat voids in closer proximity to the surfaceand larger voids were easier to detect. Thesimulations also indicated that the minimumnumber of bits of resolution required todetect most voids was within the performance

Table 1: Comparison of characteristics of a Wollaston wire probe [7] and the polyimide thermal probe.

Figure 4: (A) Schematic of a glass substrate with50-nm thick Cr lines covered by 5 µm ofphotoresist. (B) An AFM scan shows very little topographical variation. (C) The Cr lines are clearly visible with athermal probe scan. Reprinted from [15]with permission.

Performance Wollaston wire probe Polyimide probe

Tip diameter 1 µm <100 nm

Topographical resolution NA <1 nm

Temperature resolution 2.5 K <10 mK

Thermal conductance <0.23 µW K-1 <3 pW K-1

Normal spring constant 1-5 N m-1 0.1 N m-1

levels of the scanning thermal microscopy sys-tem. For example, a 100-nm diameter void at140-nm depth would require 8-bit resolutionto be detected, while a 140-nm diameter voidat the same depth would require 6 bits, and a40-nm diameter void would require 12 bits. InFigure 6, the X axis represents the ratio of voiddepth to void diameter and the Y axis the bitsrequired to detect a particular void. The twolines represent 70-nm and 140-nm depths. Ata fixed depth, the bits required to detect avoid decrease as the void size increases.

C O N C L U S I O N SThis article has reviewed a surface microma-chined scanning thermal probe that uses poly-imide as the structural material and anembedded thin-film metal resistor as the sens-ing element. The probe tip offers a diameter<100 nm, a topographical resolution of <1nm, a spring constant of <0.1 N m-1, and canbe used to detect thermal conductancechanges of the order of 3 pW K-1.

The probe was used to map the spatialchange in thermal conductance of various testsamples. Surface and subsurface characteris-tics were observed. In particular, subsurfacethermal conductance variations in copperlines have been observed. Past simulationshave predicted the feasibility of copper-voiddetection by these probes. This work reportsthe first experimental demonstration of ther-mal conductance variations in copper linesusing samples provided by Sematech.

R E F E R E N C E S1. Li, M-H., Gianchandani, Y. B. Sensors and Actuators A 104:

236–245, 2003.2. Li, M-H. et al. IEEE/ASME Journal of Micro-electro-

mechanical Systems 10(1):3-9, 2001.3. Ocola, L. E. et al. J. Vac. Sci. Technol. B, 14(6):3974-3979,

1996.4. Hammiche, A. et al. Meas. Sci. Technol., 7:142, 1996.5. Luo, K. et al. Appl. Phys. Lett. 68:325, 1996.6. Lai, J. et al. IEEE Electron Dev. Lett. 16:312, 1995.7. ThermoMicroscopes, probe model #1615-00, now part of

Veeco Probes, probe model #1615-00/1610-00/CLST-NOMB.8. Hammiche, A. et al. Applied Spectroscopy 53(7):810-815,

1999.9. Vettiger, P. et al. IBM J. Res. Develop. 44(3):323-340, 2000.10. Lerchner, J. et al. Sensors and Actuators B 70:57-66, 2000.11. Majumdar A. Scanning thermal microscopy. Ann. Rev.

Mater. Sci. 29: 505-585, 1999.12. Williams, C. C., Wickramasinghe, H. K. Appl. Phys.

Lett.49:1587, 1986.13. Gianchandani Y., Najafi, K. IEEE Transactions on Electron

Devices 44(11):1857-1868, 1997.14. Lee, J-H. et al. International Workshop on Thermal

Investigations of ICs and Systems (THERMINIC 2002),Madrid, Spain, October 2002, pp. 111-116.

15. Lee, J-H., Gianchandani, Y. B. Review of ScientificInstruments 75(5):1222-1227, 2004.

16. Basu, A. S. et al. J. Vac. Sci. Technol. B, 22(6): 3217-3220,2004.

17. Hendarto, E. et al. Proceedings 43rd Annual IEEEInternational Reliability Physics Symposium, pp. 294-299,2005.

18. Basu, A. S., and Gianchandani, Y.B. Proceedings IEEEInternational Conference on MicroElectroMechanicalSystems, Miami Beach, FL, pp 666-669, 2005.

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Figure 5: (A) SEM image of cross-section of Cu line in 60-nm dense low-k dielectric film above porous low-k. Courtesy of Sematech. (B) SEM image of trench preparation before Cu deposition. Courtesy of Sematech. (C) AFM topographical image of Cu lines under a natural oxide 300-nm wide and 125-nm thick. (D,E) Overlay thermal scans superimposed on topographical scans with <300 nm discontinuities in thermal conductance maps. These discontinuitiesoccur under the natural oxide and are not visible topographically. The area scanned was 6 x 6 µm2 and the scan rate was set at 1 Hz (D) or 1.4 Hz (E).

Figure 6: The number of bits of resolution neces-sary to detect a void at a given depth isproportional to the ratio of the voiddepth to the void diameter. These simu-lations assume that the voids exist in a150 nm x 150 nm x 280 nm feature ofthe copper, based on simulationsreported in [23].

19. Basu, A. S., Gianchandani, Y. B. Presented at the 13thInternational Conference on Solid-State Sensors, Actuators,and Microsystems, Seoul, Korea, 2005.

20. McNamara, S. et al. J. Micromech. Microeng. 15:237-243,2005.

21. Gaitas, A., Gianchandani, Y. B. An experimental study ofcontact mode scan speed constraints for polyimidecantilever probes. To be published in Ultramicroscopy,2006.

22. Li, M-H. Surface Micromachined Polyimide Scanning

Thermocouple and Bolometer Probes. PhD Thesis,University of Wisconsin, Madison, January 2001.

23. McNamara, S., Gianchandani, Y. B. Systems and Methodsfor Thin Film Thermal Diagnostics with Scanning ThermalMicrostructures. USA Provisional Patent, Application No.unassigned, Assignee: PicoCal Inc., Filed: September 2005.

24. Intel Technology Journal 6(2), May 16, 2002.©2006 John Wiley & Sons, Ltd