ULTRACOMPLIANT, PASSIVELY DECOUPLED THERMAL PROBE ARRAYS:LARGE AREA MAPPING OF NON-PLANAR SURFACES WITHOUT FORCE FEEDBACKShamus McNamara1, Amar Basu1, Joohyung Lee, and Yogesh B. Gianchandani1,*Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, USA
ABSTRACTI. INTRODUCTIONFirst introduced in 1986 [1], scanning thermal
microscopy (SThM) has found a unique place among theseveral techniques of high resolution scanning microscopy.
It permits mapping of topography, temperature, thermal
conductivity, thermal capacitance, and performing microcalorimetry with sub-100 nm spatial resolution [2]. The
scanning probe has a thermal sensor (typically a bolometer)
at a sharp tip located at the end of a cantilever. Scanning isbest performed with the probe in contact with the sample toeliminate the high thermal resistance of an air gap, but thisconventionally requires a mechanical feedback loop to
prevent the probe from scratching the sample. As in atomicforce microscopes, the contact force is sensed by measuringprobe deflection with a reflected laser. The desire to increase
throughput in scanning microscopy (both SThM and AFM)
has prompted the design of arrays in which multiple probe
tips scan in parallel [3-8]. With arrays, however, the problemof feedback becomes a difficult issue because parallel
scanning requires each cantilever to have its own addressable
feedback loop. Individual actuation of cantilevers has been
explored, including piezoelectric films [3, 6], and thermalbimorphs [8]. While these approaches are effective forlimited variations in topography, they do not easily
accommodate samples with micron level topographicalvariation, such as integrated circuits or biological cells.
*Bridge

LNA

LPF DAQePSPD

PrerLasThis paper describes an 8-probe system for scanningthermal microscopy. The design of the probe array, alongwith the choice of polyimide as the structural material
provides very large compliance that virtually eliminates theneed for z-axis mechanical feedback both at the chip andprobe level in contact mode scans. The high complianceaccommodates significant variations in the sample surface,and also prevents damage to soft samples. In addition, sinceintegrated actuators and accompanying circuitry are nolonger required, the prospect of scaling to large numbers ofprobes for high speed, high resolution thermal mapping oflarge areas with simple detection circuitry is enhanced.
Based on single probes fabricated in the same process, theestimated spatial resolution, thermal conductance resolution,and temperature resolution are 50 nm, 11 pW/K, and 1.2 mK
respectively. Scalability and performance of the 8-probe
prototype are evaluated, addressing issues of speed vs.
resolution, thermal and mechanical decoupling. Results
demonstrate that contact mode scans can provide better than2 µm spatial resolution at speeds greater than 200 µm/sec
with each probe.

ob1

XY Micromanipulator

XY ScanController

ComputerInterface
Z Axis Actuation Not NeededFig. 1: Z-axis feedback and integrated actuators are notrequired in the ultracompliant microprobe array presented
here. A simple open loop interface circuit is sufficient tobias the probe tip temperature and read the temperature via
changes in probe resistance.

Furthermore, all types of integrated actuators require
additional fabrication steps, control circuitry, and electricalinterconnect, thereby increasing manufacturing andcalibration complexity.
In this work we present an ultracompliant probe arraythat can be scanned across a sample with minimum force
without mechanical feedback. The use of polyimide as thecantilever structural material, previously reported by ourgroup, offers not only a high thermal isolation, but a very
high compliance [9], resulting in a 10-100x reduction incontact force over conventional materials like silicon and itsdielectrics. Consequently, the contact force for eachcantilever in the array remains low over a wide range ofdeflections, eliminating the need for probe level feedback
typically required to prevent damage to the sample and thetip. In addition, the structure can be designed in a mannersuch that the cantilevers are decoupled both thermally andmechanically, enabling high-speed parallel thermal mappingof samples with large topographical variation using simple
detection circuitry and no integrated actuators (Fig. 1).
II. DEVICE STRUCTURE AND OPERATIONThe structure of the probe array is shown in Fig. 2.
Eight cantilevers extend off a single shank overhanging fromthe edge of the die. To scan a sample, the die is inverted andmounted on a motorized XY scanning stage (Fig. 1). Eachprobe in the array operates as a typical microbolometer. A
thin film metal resistor on the probe tip is used to both heat
the tip and detect temperature variations during the scan.
The probe is connected to a Wheatstone bridge that provides
bias current and senses changes in resistance. A low noise
amplifier and low pass filter are used to minimize noise. A
PC interface is used for data acquisition and to control theposition of the XY scanning stage. As mentioned previously,

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SiO2 tip sharpen silicon nitride
Si substrate(a)notch for tip

Resistive materials(d)upper polyimide

sacr. material

(b)

(e)
lower polyimide

Fig. 2: SEM of the thermal probe array after assembly.
Individual cantilevers protrude from a single shank. The
entire array is flipped over and held in place in one step.
nt)
mm Probeon
Be Shanam kSegmeProbe n+2Ks

K1

K2 ...

KnF2FnSample(a)(b) F1
Fig. 3: (a) Schematic showing the conceptual operation of
the probe array over a sample with large topographicvariation. Each probe deflects vertically without affecting
the other probes. (b) First order mechanical model of thearray structure. For effective mechanical decoupling, thespring constant of the shank KS should be large compared tothe cantilever spring constants K1, K2, etc.

the low spring constant of the polyimide probes, typically 10100x lower than conventionally used, enables scanning mostsamples in non-feedback contact mode without damage.
Mechanical operation of the array is depicted in Fig 3a.
Ideally, each probe deflects without affecting others. During
fabrication however, since all the probes are simultaneously
flipped using a common segment of the beam for handling,some degree of coupling is inevitable. Intuitively, the system
can be thought of as a simple mechanical network shown inFig. 3b, with an array of springs representing individual
cantilevers coupled to a single spring representing thecommon beam segment. The precise value of mechanicalcross-talk between beams depends on a number of factors,
including the design of the beams (i.e. the length and widthof the separated segments and the common segments), thepre-deflection of the beams in contact, and the magnitude of
attractive surface forces that exist between each tip and thesample. From the viewpoint of the designer, however, it isbest to increase the lengths of the separated segments andminimize that of the common segment.
Thermal coupling between adjacent probes can bedefined as ∆t P1 /∆t P2 , i.e. the temperature shift in a probe
caused by a shift in the adjacent probe. Finite element
analysis was conducted in ANSYS, ignoring convection andassuming that the array is held 15 µm above a thermallyconductive substrate. The primary path of heat exchange

Au(f)

(g)
Probe nProbe n+1

(Co(c)Au

Au thermo-compression
bondFig. 4: Process flow for fabricating the polyimide based
thermal probe array, suitable for post-IC processing. A tip
notch is created, followed by a sacrificial layer, a resistor
sandwiched between two layers of polyimide, and a goldlayer for thermo-compression bonding. After etching thesacrificial layer, the probe is flipped, resulting in a probe
overhanging the edge of the die.
between adjacent probe tips was found to be lateral
conduction through air, and the coupling ratio is –24 dB.
The factor improves by 10x if the distance between probe
tips is doubled to 170 µm.
III. FABRICATIONThe ultracompliant probe array is fabricated in a lowtemperature 7-mask process suitable for post CMOS
fabrication. Shown in Fig. 4, the process consists primarilyof surface micromachining with an additional
thermocompression bonding step.
A mold for the tip structure is first created by anisotropic
etching of silicon followed by an oxidation step for tip
sharpening (Fig. 4a). Sacrificial titanium deposited below
the cantilever structures facilitates release of the cantilevers

Fig. 5: Photograph of the thermal probe array before the flipover step. The interconnect is a thick metal line to reduce
the series resistance to the thin-film bolometer. The twogold regions mate when performing the thermo-compressionbond.

Fig. 6: Reconstruction of a thermal
image by combining overlapping scanregions. Shown here is a 750 µm x 200µm area on a commercial IC containing
60 µm x 85 µm bonding pads with 120µm pitch.
(Top) Thermal images
obtained from four probes spaced 170µm apart scanning at 25 µm/sec.
(Middle) Optical image. (Bottom)
Horizontal line scan taken with asurface profilometer showing 2 µm
topographical features between bondpads. In this scan, the thermal image
conveys primarily topographicalinformation, with sensitivity of 12mV/µm, dynamic range greater than 2µm, and a noise limited minimum
detectable topography of 80 nm.
(Fig. 4b). The cantilever itself is formed by two polyimidelayers (HD Microsystems P2613) with two metal layers
embedded within. The first layer of polyimide is etched fromthe tip region to permit the metal tip (to be formed next) tocontact the sample (Fig. 4c). A thin metal layer is thendeposited in the mold to form both the metal tip and theresistor. A second, thicker metal layer forms electricalinterconnection to the probe tip (Fig. 4d). The top layer ofpolyimide is used to insulate the metal and provide strength
to the probe tip (Fig. 4e). Thick gold patterns are deposited
after the second polyimide layer for thermocompression
bonding (Fig. 4f). After release of the cantilevers in HF, theprobe shank is flipped over and bonded at 200° C (Fig. 4g),
resulting in a probe shank overhanging the edge of the waferdie (Fig. 2).

approximately 12 minutes. However, to put this inperspective, it is also necessary to consider dynamic range,
because most high throughput micro-probe systems [4,6] are
not intended for mapping samples with wide surface
variations.
Dynamic range and topographical resolution isdemonstrated from the line scans in Fig. 7. Because thisstructure is coated with a thin gold layer of uniformthickness, it is expected that the thermal signal obtained will
convey primarily topographical characteristics. The signal
obtained is approximately linear with respect to topography.
Using a 12 mA bias current, a 90 mV contrast is obtained
over a 7 µm topographical variation, indicating a sensitivityof 12 mV/µm. Observed noise voltage is 1 mV, translating
to a minimum detectable signal (MDS) of approximately 80

IV. EXPERIMENTAL RESULTS AND DISCUSSIONGoldPolyimide
Silicon

7 µm
4 µm 10 µm

20 µm

10 µm 4 µm

2 µm
FeatureDektak
10 µm/sNormalized Voltage

To validate the functionality of the probe array, a 750µm x 200 µm area was scanned on a commercial IC
containing bond pads. Four of the eight probes in the array
were used, each generating a 200 µm x 200 µm image
consisting of 100 lines scanned at 25 µm/sec. Raw data was
subject to standard signal processing to remove scan artifacts
such as linear trends, and filter out high frequency noise.
Scan results (Fig. 6) illustrate how a thermal image may bereconstructed by merging overlapping scan regions. All
measurements made over the 2 µm of topographical variation
were obtained without force feedback.
With throughput being the main motivation behindarrayed probes, it is of interest to gauge the scan speed atwhich features can be resolved. Two 7 µm deep
anisotropically etched silicon trenches coated with polyimide
and gold were scanned at increasing speeds. Line scans inFig. 8 reveal an observable feature of about 2 µm can beresolved at speeds up to 200 µm/sec. (In fact, 10 µm features
can be resolved even at 2 mm/sec). The effective scan speed,which scales with the number of probe tips, is 1600 µm/sec
for the 8 probe prototype. This suggests that a 1.5mm x1.5mm IC with 2 µm minimum features can be imaged in

20 µm/s
50 µm/s
50Hz
Resonance

100 µm/s
200 µm/s
500 µm/s
1000 µm/s

02550
Distance (µm)

75Fig. 7: Speed scaling study. Two 7 µm pits coated withpolyimide and gold were scanned at increasing speeds. A 2µm feature (labeled) is observed at speeds up to 200 µm/sec.
Mechanical resonance at 50 Hz was observed at speeds
above 100 µm/sec.

Probe 1 Start
Probe 2 Start
6 µmCu(b)Voltage (mV)

Voltage (mV)

(a)
20Cu

V. CONCLUSIONS

Probe 10-20020200

400600Probe 20-20(c)Polymer
Coating0

not identical (variation in tip resistance and tip wear cancause difference in the obtained signal), it is clear that themovement of probe 2 into the basin does not affect the signalof probe 1. The line scan as well as the 2-dimensional maps
support a strong degree of mechanical decoupling over a 6µm topography.

200X (µm)

400600This effort has demonstrated that a mechanicallydecoupled 8-probe array can be used for parallel, highresolution thermal mapping of features with large
topographical variation without force feedback. Eliminationof feedback is enabled by using ultracompliant probes thatminimize contact force, preventing damage to both the tip
and fragile samples. In non-feedback mode, 2 µm lateralfeatures were resolved at speeds up to 200 µm/sec per probe,and 10 µm features can be resolved even at 2 mm/sec. Noise
limited minimum detectable signal (MDS) for topographicalvariations was found to be 80 nm, and the dynamic range is >7 µm. If feedback is used, single probe versions developed
previously by the same technology [9] obtain an MDS of 1nm.
ACKNOWLEDGEMENTS
The facilities used for this research include the SolidState Electronics Laboratory at the University of Michigan.
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