Scanning thermal lithography: Maskless, submicron thermochemicalpatterning of photoresist by ultracompliant probes
Amar S. Basu,a) Shamus McNamara, and Yogesh B. Gianchandani
Department of Electrical Engineering, University of Michigan, 1301 Beal Avenue, Ann Arbor, Michigan
48109(Received 3 June 2004; accepted 30 August 2004; published 10 December 2004)
This article introduces a scanning probe lithography technique in which ultracompliant thermalprobes are used in the selective thermochemical patterning of commercially available photoresist.
The micromachined single-probe and multiprobe arrays include a thin-film metal resistive heaterand sensor sandwiched between two layers of polyimide. The low spring constant 共⬍0.1 N / m兲 andhigh thermal isolation provided by the polyimide shank is suitable for contact mode scanning across
soft resists without force feedback control. The probes provide what is effectively a spatiallylocalized postexposure bake that crosslinks the photoresist in the desired pattern, rendering it
insoluble in developer. For 450-nm– 1400-nm-thick AZ5214E (Clariant Corp.), line and dot featureswith sizes of 450 – 1800 nm can be printed using probe powers of 13.5– 18 mW, and durations of
1 – 60 s per pixel. Variation of feature sizes with process parameters is described. © 2004 American
Vacuum Society. [DOI: 10.1116/1.1808732]

I. INTRODUCTION
With the increasing costs of photomasks—often in excess
of one million dollars for state-of-the-art complementary
metal-oxide-semiconductor processes—alternatives to optical lithography have been sought, particularly for lowvolume manufacturing and prototyping in conventional processes. As a maskless lithography approach, scanning probe
lithography (SPL) offers the promise of high spatial resolution (which is not limited by the diffraction of light) and, forlow-volume applications, substantial savings in mask andequipment costs.
It is interesting to note that as methods in scanning probe
microscopy developed over the last few decades, most havebeen applied to lithography.1 Years after the invention of thescanning tunneling microscope (STM) in the 1960s, scanning probe lithography was born when an STM was later
used to selectively oxidize hydrogen-terminated Si surfaces
with nanometer resolution.2 Likewise, the atomic force microscope (AFM), originally used for mapping topography,
was later modified to “scratch” thin metals3 and softpolymers,4 again with resolution in the tens of nanometers.
The near-field scanning optical microscope, which employs
scanning probe tips to focus light, was used directly to optically pattern photoresist with resolutions beyond the far-field
diffraction limit.5 Several other lithographic techniques havegrown out of AFM and STM technology, two of which areelectrostatic6 and dip-pen lithography.7
Scanning thermal microscopy, introduced in 1986, hasalso found complementary efforts in lithography, but nearly
all are based on thermomechanical indentation, that is, theuse of heat and pressure in submicron tips to etch pits intopolymers with glass transition points. Initally, heat was provided by an external laser pointed at a conventional AFM
a)Electronic mail: basua@umich.edu

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J. Vac. Sci. Technol. B 22(6), Nov/Dec 2004

tip,8 and later by thin-film heaters embedded in the
cantilever.9 Thermomechanical indentation has been pursued
with interest in ultra-high-density data storage, the most notable device being the IBM “millipede.”10 In this work, wedemonstrate that scanning thermal probes can be applied towards lithography by thermally catalyzing a chemical reaction in a submicron region (Fig. 1) rather than by thermomechanical indentation. We have demonstrated this principle on
the positive tone photoresist AZ5214E (Clariant Corp.) in aprocess similar to image reversal. The use of localized heatto pattern this resist has been previously explored by focusing semiconductor lasers.11 Some potential advantages of the
proposed method are that it is less prone to tip wear andproduces less debris when compared to purely mechanical
SPL techniques; however, it does require scanning over soft
photoresist. An enabling device for this technique has been
the ultracompliant probe technology, reported previously
by our group,12,13 which minimizes damage to both thesample and the probe tip. Both single probes and eight-probearrays are shown to provide thermochemical patterning ofphotoresist.
II. PROCESS AND DEVICE CONCEPTS
Several structural features of the ultracompliant probes(Fig. 2) make them well suited for the lithography process,the most important aspect of which is use of a thin-film
polyimide as the structural material for the cantilevers. The
excellent thermal resistance of the polymer allows the probe
tips to be heated to temperatures of nearly 300 ° C by embedded thin-film heaters operating at ⬍20 mW, while thelow spring constant (⬃0.08 N / m for the single probe) allows
for scanning over photoresists and other soft polymers in theabsence of mechanical feedback without scratching the resist
and with minimal wear to the tip. The elimination of force
feedback is especially important when scaling to arrays ofprobes. If rigid cantilevers are used, tip height nonuniformity

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Basu, McNamara, and Gianchandani: Scanning thermal lithography: Patterning of probes

FIG. 1. Process flow for scanning thermal lithography of AZ5214E resist. (a)Flood exposure generates photoacids. (b) Spatially localized thermal
crosslinking occurs under the heated probe tip (inset) as a result of thephotoacids and elevated temperatures. (c) The insoluble crosslinked region
remains after development in TMAH developer.

across the array can result in significant variation in contact
force when it is lowered to the substrate,10 necessitating
feedback control at the probe level to prevent excessive force
in any one tip. The low spring constant in the ultracompliant
array passively accomplishes the same function, thus eliminating the need for force feedback. An additional feature of
the array is that each probe has individual heater control
which can allow for independent patterning if desired.
Both single probes and multiprobe arrays are fabricated in
the same low-cost micromachining process in which a thinfilm metal bolometer is sandwiched between two insulating

FIG. 2. (a) Schematic of the micromachined single probe, and (b) scanningelectron micrograph of the eight-probe array surface micromachined frompolyimide on a Si substrate. Thin-film resistors integrated on the tip of thecantilevers allow the individual probes in the array to be heated independently of one another.
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layers of polyimide.13 The thin metal film, consisting of
300– 1200 Å Au between two adhesion layers of 100 Å Cr,
is molded into an anisotropically wet-etched notch in the Si
substrate to form the scanning tip. Two lines extending outwards from the tip region form the heating element. Electrical connection to the bolometer is provided by thicker metallines extending the length of the cantilever. The total resistance ranges from 20 to 200 ⍀ depending on design and process parameters. The probe is then released from the substrate, flipped out over the die edge, and held in place by athermocompression bond between two thin films of Au.
With regard to the patterning of the sample by the probes,
the photoresist AZ5214E is one of the very few positive tone
photoresists that can function as a negative tone resist by aprocess called image reversal. Conventional image reversal
begins with a patterned UV exposure which generates photoacids in unmasked regions. A subsequent postexposurebake (PEB) thermally activates a crosslinking reaction catalyzed by the photoacids in the previously exposed regions.14
The crosslinked regions become insoluble in alkali developer, and are insensitive to any further UV processing. The
substrate is then flood exposed to solubilize all noncrosslinked regions. Development in tetramethyl ammoniumhydroxide (TMAH) leaves a negative photoresist image.
III. EXPERIMENTAL RESULTS
Temperature calibration of the single probe is relatively
straightforward because the fractional change in the probe
resistance 共⌬R / R兲 is directly proportional to the increase in
tip temperature 共⌬T兲 by a constant K.12 The thin-film heater
accounts for the majority of the probe’s resistance and results
in a linear relationship between the measured resistance andtip temperature. To determine the constant K, the singleprobe cantilever was immersed in water and the current was
ramped until bubble formation and rapid evaporation wasobserved. Water was found to boil at a 2.16% increase inprobe resistance, and by correlating this to a 75 ° C increasein tip temperature, K was found to be 288 ppm/ ° C. Repeating the same experiment with isopropyl alcohol (boilingpoint 85 ° C) resulted in a virtually identical figure. Using theabove calibration, the tip temperature can be correlated to themeasured resistance during probe operation. The probes canbe heated to approximately 300 ° C before high temperatures
begin to damage the cantilever structures. It should be notedthat the temperature versus power relationship differs depending on the degree of thermal isolation from the surrounding environment. A probe operating in proximity or incontact with the substrate requires about 30% more power
than when suspended in air due to the heat loss to the substrate. Nevertheless, the typical input powers needed to reachtemperatures in excess of 250 ° C while in contact was⬍20 mW.
The scanning thermal lithography (Fig. 1) proposed forthis effort is similar to the conventional process, except thatit involves patterning the PEB instead of the initial exposure.
AZ5214E photoresist at full concentration was spun on Si
wafers to obtain a thickness of 1.4 ␮m, and diluted 50% with

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FIG. 4. (a) Finite element simulations with photoresist thicknesses of
1.4 ␮m, 450 nm, and 100 nm show that the geometry of heat flow becomes
vertical as the photoresist is thinned below the diameter of the tip, resulting
in smaller features. (b) Dots obtained after fully developing 1400 and
450 nm resists follow the same trend. The roughened surfaces in the micrographs are due to an Au/ Pd layers sputtered for scanning electron
microscopy.

FIG. 3. Dot sizes can be controlled by the heating times as well as heating
power. (a) Reduction in size with shorter heating times using a probe biased
at 18 mW on 450 nm resist. (b) Dot sizes vs heating time for 18 and13.5 mW power.

polyethylene glycol methyl ether (PGMEA) solvent (Nano
EBR, Clariant Corp.) to obtain 450 nm thickness. Further
dilution resulted in poor uniformity; however, in general it isexpected that thinner resists will permit smaller feature sizes
to be patterned (discussed later in this article). After softbaking, the resist was flood exposed to generate photoacids globally. The heated thermal probes were then scanned acrossthe surface point by point, each point being heated anywhere
between 1 and 60 s. Regions under the probe tip crosslink
and become alkali insoluble. The resist was developed indiluted TMAH 共0.21 N兲 in order to maintain the highest contrast between crosslinked and noncrosslinked regions.15
To fully crosslink, AZ5214E requires temperatures ofabout 90 ° C, but it was found that simply biasing the probe
tip at this temperature was inadequate due to the thermal
contact resistance between the tip and the photoresist. Except
where indicated, the single probe was biased at 18 mW, corresponding to a tip temperature of 275 ° C. In addition, thesubstrate was biased at 45 ° C using a hot plate in order tofurther reduce the amount of heat that the probe needed toprovide. Tests showed that when the contact force was madeexcessively high by flexing the cantilever towards the substrate, the increased pressure in combination with the hightip temperature resulted in thermomechanical indentations
penetrating all the way through the resist, and no thermal
patterns were observed after development. Conversely, weakcontact forces resulted in poor thermal contact, also resulting
in no pattern formation. An intermediate contact force of
JVST B - Microelectronics and Nanometer Structures

about 480 nN was found to be successful. The advantage ofusing flexible probes is again noted here in that once theprobe is biased at some contact force, that force does notchange significantly over the sample topography, thereby increasing the uniformity of the patterned structures.
Using the process described above, submicron dots andlines were obtained, in some cases when the photoresist waspartially developed, and in other cases when it was fully
developed. It was found that dot sizes could be well controlled by the changing the probe temperature as well as theheating times (Fig. 3). Using an 18 mW bias on 450 nm
resist resulted in features that ranged from 400 to 1800 nm
as heating times were increased from 1 to 60 s. Reducing
the power to 13.5 mW power resulted in the same featuresizes, but required longer heating times. The above results
were found after partially developing the 450 nm photoresist.
Features patterned at 18 mW for longer than 30 s remained
after full development of both 1.4 ␮m and 450 nm resists(Fig. 4). Biasing at high powers and heating for longer times
results in a higher degree of crosslinking because thecrosslinking reaction is, in effect, both temperature and timedependent.
Feature sizes shrink with photoresist thickness due to thedecreased lateral flow of heat. Two-dimensional ANSYS heatflow models [Fig. 4(a)] qualitatively show that as the thickness of the resist is reduced below the diameter of the scanning tip, the cylindrical flow of heat results in features ofapproximately the same size as the tip. A thick resist permits
the heat to spread laterally, increasing feature sizes. For example, using identical heating powers and times (18 mW,
30 s) on both 1.4 ␮m and 450 nm resists produced dot sizesof 590 and 450 nm respectively, after full development of the3220

Basu, McNamara, and Gianchandani: Scanning thermal lithography: Patterning of probes

FIG. 5. (a) and (b) Lines of 500 nm width written by patterning successive
dots with 200 nm spacing (1 s heating per dot) can be seen after partial
development of the photoresist. (c) and (d) Parallel lithography is demonstrated in which two adjacent probes in the eight-probe array simultaneously
generate the same pattern.

photoresist [Fig. 4(b)]. The use of a thick resist also runs therisk of creating reentrant cross-sectional profiles that can beundercut during development.
Fully developed dots of 450 nm minimum diameter wereobtained with photoresist layers of approximately 450 nm
thickness. Further scaling down of features sizes for the purposes of lithography can be accomplished by using sharper
tips and thinner resists, particularly those that are chemically
amplified (chemical amplification can also help to reduceheating times and increase throughput). The diameter of thescanning tips in the present design can be made as small as
50 nm using oxide sharpening,16 and others have reported
custom fabrication of thermal probes with tip sizes as small
as 20 nm.17
Line patterns formed by heating successive pixels areshown in Fig. 5(a). Linewidths of 500 nm were observed inpartially developed resist at effective speeds of 200 nm/ s,
while slower speeds of 10– 20 nm/ s were needed to retain
the lines after full development. This is likely because astronger degree of crosslinking was required to prevent undercutting of the resist. The torsional flexing of the compliant
cantilever resulted in some line distortion [Fig. 5(b)], but thiscan be minimized by lifting the probe between each pixel.
The structure of the probe itself could also be modified toprovide a stiff torsional spring constant while maintaining a
low vertical spring constant.
The final experiment demonstrated the use of the multiprobe array to perform parallel lithography. The eight-probearray was used to write pixels at 5 ␮m pitch, and the patterns
produced by two adjacent probes are shown in Figs. 5(c) and
5(d), respectively. Probe tips were lifted up between pixels,
thereby reducing the line distortion; however, during thecourse of this experiment, external vibrations shifted some ofthe probes, resulting in double patterning in some areas. With
this exception, the feature sizes are fairly uniform between
probe tips. Although not shown in this experiment, theprobes can also be operated at different powers, allowing theformation of independent patterns.
During the course of the experiments, reliability of theprobes was found to be excellent in that all of the probes
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appeared to run many samples with no noticeable tip damage
or deterioration in thermal performance. Longevity of scanning probe tips is an important aspect to consider if they are
to be used in a manufacturing process.
In conclusion, this effort has demonstrated that scanningthermal probes can be used, both singly and in parallel, in thethermochemical patterning of AZ5214E. The resolution ofthis technique can be scaled down in a relatively straightforward manner. The lessons of this effort are as follows: first,
the throughput can be addressed to some degree by the use ofmultiprobe arrays; second, pixel sizes can be controlled byheating times and powers, and finally, the procedure is lesscostly in the sense that it does not require photomasks or
optics, nor does it require a vacuum or UV filtered environment. Ultimately, the resolution and throughput of this technique are determined by the size of the tips, the number ofprobes that can be scanned in parallel, and the thickness andthermal sensitivity of the resist. In a broader sense, this work
has shown that scanning thermal lithography can be used forpatterning any thin film for semiconductor and biologicalapplications or for triggering a chemical reaction at the surface of a material.
ACKNOWLEDGMENTS
This effort was supported in part by the Whitaker Foundation (A.B.).
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