ARTICLE IN PRESS

Ultramicroscopy 106 (2006) 838–846
www.elsevier.com/locate/ultramic

Microfabricated torsion levers optimized for low force andhigh-frequency operation in ﬂuids
Arthur Beyder, Frederick Sachs
Department of Physiology and Biophysical Sciences, Center for Single Molecule Biophysics, State University of New York at Buffalo,
3435 Main Street, 301 Cary Hall, Buffalo, NY 14214, USA
Received 3 September 2005; accepted 30 November 2005

Abstract
We developed a mass production fabrication process for making symmetrically supported torsion cantilevers/oscillators with highly
compliant springs. These torsion probes offer advantages in atomic force microscopy (AFM) because they are small, have high opticalgain, do not warp and can be made with two independent axes. Compared to traditional AFM cantilevers, these probes have higher
frequency response, higher Q, lower noise, better optics (since the mirror does not bend) and two data channels. Soft small levers with
sub-pN force resolution can resonate cleanly above 10 kHz in water. When fabricated with a ferromagnetic coating on the rigid reﬂecting
pad, they can be driven magnetically or serve as high-resolution magnetometers. Asymmetric levers can be tapping mode probes or highresolution accelerometers. The dual axis gimbaled probes with two orthogonal axes can operate on a standard AFM with single beam
illumination. These probes can be used as self-referencing, drift free, cantilevers where one axis senses the substrate position and the otherthe sample position. These levers can be optimized for differential contrast or high-resolution friction imaging.
r 2006 Elsevier B.V. All rights reserved.
PACS: 07.79.Lh; 07.79.Pk; 07.79.Sp
Keywords: Atomic force microscopes; Magnetic force microscopes; Friction force microscopes; Microfabrication; Force; Friction

1. Introduction
The atomic force microscope (AFM) has provided theability to measure and manipulate matter at the atomic
level [1]. The core of the instrument is a compliant
cantilever whose bending generates the output. To ﬁrst
order, the bending is a linear function of the force appliedby the sample. Typically, cantilever deﬂection is monitoredby an optical lever, a laser beam reﬂected off the cantilever
onto a position sensitive photodetector (PDT) [2].
The resolution of an AFM is limited by the physical/
mechanical properties of the cantilever [3]. The scanning tip
shape is convolved with the sample and thus determines thespatial resolution [4]. The force/distance resolution islimited by the thermal ﬂuctuations of the cantilever,Corresponding author. Tel.: +1 716 829 3289; fax: +1 716 829 2569.

E-mail address: beyder@gmail.com (A. Beyder).
URL: http://www.sachslab.buffalo.edu.
0304-3991/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultramic.2005.11.014

1/2 kBT for each degree of freedom [5]. The apparent
compliance of a sample is a convolution of the complianceof the cantilever and that of the sample itself. Generally,
soft cantilevers are preferred when measuring force
and stiff cantilevers are preferred when measuring
displacement [6].
The AFM has a particular appeal to biologists because it
can be used on live samples under water, with the potential
for atomic spatial resolution, examination of the forces
produced by single molecules, and a time resolution
approaching microseconds. However, in liquids, thecantilever behavior is signiﬁcantly degraded by viscousdrag [6]. Drag reduces the frequency response and the force
resolution. Since the drag is a function of the object’s size[7], smaller probes are better.
The ﬁrst cantilevers were made from a gold foil with adiamond shard as a tip [1]. Later, silicon micromachining
permitted mass production with well-deﬁned mechanicalproperties [8,9]. Current commercial cantilevers are most

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commonly made from silicon (Si), silicon nitride (stoichiometric—Si3N4 or low-stress silicon-rich—SiN) andsilicon dioxide (SiO2) [8]. The stiffness of the cantileveris a cubic function of length and an inverse cubic ofthickness; high compliance (soft) levers are usually
made by making levers longer [10], but long cantilevers
have large surface areas, thus they are highly damped inliquid. One can also make cantilevers soft by making
them thin. Crystal silicon cantilevers are generally thick(41 mm) and stiff [11]. However, using thin uniform
ﬁlms (o1 mm) of low stress of SiN, it is possible tofabricate much softer levers [12]. All commercial cantilevers are asymmetrically supported and usually metal
coated for reﬂectivity making them prone to warping fromresidual stress [13] and as a thermal bimorph, changes intemperature increase the warp [14].
To improve the cantilever response in ﬂuid, theHansma group made small cantilevers [15–18], as thin as
86 nm and 27 mm long [19]. However, that design had anumber of practical problems. Due to their small area,
there was limited access to the sample and limited target
size for the optical beam. These cantilevers could not be
used on standard AFMs because they required highaperture lenses to focus the beam on the cantilever [18].
Furthermore, high aperture lenses have a short working
distance so that there is little room to introduce otherobjects such as micropipettes near the sample [20]. The
small design does not resolve the general problem ofcontinuous beam levers, that bending disperses the opticalbeam [21].
We have taken a different approach to making
small AFM probes using symmetrically supported
torsion levers. The lever itself, containing the tip, is a stiff
silicon (Si) pad 20  20  5 mm that is a convenientmirror size for standard AFM optics. The padoscillates on compliant torsional hinges, which are madeof SiN. The levers are located at the end of stiff Si
extension beams that hold them away from the die,therefore increasing accessibility to the sample and opticalclearance.
Torsion actuators and sensors are common mechanisms
in micro-electromechanical systems (MEMS) [22–24].
Torsion oscillators are generally made from singlecrystal silicon, and have served as movable mirrors,
tunable capacitors, RF switches, gyroscopes and angularrate sensors [25]. Oscillators made from Si are stiff
unless the fabrication is done with sub-optical resolution[22] or the beams are long [26], making them drag-prone. If
the oscillators are made from softer materials, such as
silicon nitride, they suffer from ﬂexure of the entire
assembly, including the support arms and the torsion pads
themselves [21]. By using two different materials, thesupporting structures can be stiff and the hinges soft. We
developed a process to make such composite probes in a
mass production environment, while retaining the ﬂexibility to make a variety of small probes with custom
geometry.

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2. Methods
2.1. Device fabrication
The fabrication process can be divided into three phases:(1) a bottomside etch to produce a thin silicon membrane
and to deﬁne the support chip, (2) direct fabrication of thetip, and (3) topside patterning of the stiff silicon elements
and the thin ﬁlm hinges. The process is summarized inFig. 1.
We ﬁrst produce the thin silicon membrane using
aqueous potassium hydroxide (KOH) to etch the bottomside of /1 0 0S Si wafers (1a). The etch proceeds most of
the way through the wafer, leaving at the topside amembrane that is stiff by comparison to the hinges. Thisetch step utilizes a new procedure to produce optically ﬂatmembranes of controlled thickness [27].
In the next step (1b), we deposit a SiO2/SiN thin ﬁlm
stack using low-pressure chemical vapor deposition(LPCVD). We use high-quality, low-stress, silicon-rich
SiN ﬁlms [28]. This bottomside SiN layer is later patterned
into the hinges, while the SiO2 layer serves as protection
until the ﬁnal release.
The following steps are performed on the topside of thewafer to make the tips and pattern the cantilevers. Since wehave SiO2 and SiN and photoresist available for masking(2a), there are several methods available for making thetips [8,11,29,30]. Step 2b is a controlled undercut of a small
masking island to produce a Si tip. We have produced
high-aspect atomically sharp tips by undercutting LPCVD
SiO2 with SF6. We have also made lower-aspect tips using

Fig. 1. Microfabrication sequence.

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SiN as a mask and KOH for an anisotropic undercut. Oncethe tip is produced, it is protected through the rest of theprocess by a thin layer of plasma-enhanced chemical vapor
deposited (PECVD) SiO2 (2c).
After making the tip, we deﬁne the hinge and theoscillator. We use a double mask of PECVD silicon dioxideand photoresist to complete production of a four-levelstructure. The opening for the hinges is patterned into the
SiO2 layer (3a), which is then covered by a new photoresist
layer. Next, the cantilever shape is imaged onto thephotoresist and etched into the silicon dioxide layer (3b).
We etch the silicon around the cantilever using theinductively coupled plasma reactive ion etch (ICP-RIE)
C4H8/SF6 [31]. Then another RIE removes the underlying
SiO2 and SiN layers, suspending the cantilever (3c).
Finally, the photoresist layer is removed, exposing thepreviously patterned SiO2 layer. Again, we use ICP-RIE torelease the hinges (3d). To release the nitride hingelayer and to uncover the tip, which until now has beenprotected by SiO2, we selectively remove the SiO2 using
10–50% HF (3e).
At this point, we can deposit a metal layer to increase
reﬂectivity. The 5-mm-thick silicon pads are reasonably
reﬂective, but we also deposit 50 nm Au to increase
reﬂectivity [27]. We have also deposited other metals,
such as Co, to produce magnetic levers for MAC mode
AFM [32].
2.2. Evaluation of levers’ dynamic performanceFig. 2. (A) SIMODE simulation of the fabrication of the handle (etched in
40% KOH at 80 1C). The chip is attached to the wafer at the lower edge.
(B) SEM of a fabricated handle chip.

For small deﬂections, the lever can be treated as a simple
harmonic oscillator with one degree of freedom. The
oscillator is continually excited by thermal energy and thisenergy may be used to calibrate the oscillators’ spring
constants [33,34]. For calibration, we use a commerciallyavailable Quesant Nomad AFM [35]. Calibration requires
two steps. In the ﬁrst step, optical sensitivity of the PDT isdetermined by using a calibrated PZT to gently press theprobe against a stiff substrate until a desired deﬂection isreached. This calibration is used to convert the voltage

Fig. 3. SEM images of integrated tips. (A) SF6 RIE, (B) high-magniﬁcation view of A, (C) KOH tip, (D) high-magniﬁcation view of C.

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output from the PDT into z-displacement. In the second
step, a time trace of the thermal noise is acquired (National
Instruments A/D, PCI NIDAQ). An FFT of this data
provides the power spectrum, which identiﬁes the resonantpeak of the probe. The peak is ﬁt to a Lorentzian whose
integral is the mean position noise. Thus, the value of thisintegral (P) provides a direct estimate of the spring
constant k ¼ kBT//z2S ¼ kBT/P, where kB is Boltzmann’sconstant and T is temperature. Once the spring constantand sensitivity are calibrated, we can measure the noise of
the lever in the bandwidth of interest.

3. Results and discussion

841

additional advantage of using the KOH etch to produce thechip handle is that the chip handles have a sloped front
end, increasing the optical clearance (Fig. 2B). A common
problem for KOH-assisted deﬁnition of silicon blocks isconvex corner undercutting. During the etch, low-density,
high-etch-rate Miller planes are exposed resulting insigniﬁcant rounding [36,37]. This undercut is critical if
the etch is deep, as is the case in the through-the-wafer etch(450 mm) that we use. With the help of commerciallyavailable KOH etch simulation software (SIMODE, http://
www.infotech.tu-chemnitz.de/wetel/ensimode.htm), wewere able to design a set of compensation structures atthe corners that allowed production of nearly rectangularchip handles sized to ﬁt standard AFM holders(1.8 mm  3 mm) (Fig. 2A).

3.1. Fabrication—features of the fabricated oscillators
3.1.1. Chip handles
We use the anisotropy of the KOH etch to produce chipsthat are solidly supported by the wafer throughout
processing and can be easily removed from the wafer. An

Fig. 4. (A) Hinges are produced as SiO2/SiN stack. (B) HF may be used to
produce hinges as thin as 10 nm (hinge imaged obliquely).

3.1.2. Silicon membranes
The Si membranes that are produced in Phase 1 are used
to fabricate the tip and other stiff parts of the levers.
Membranes cannot be arbitrarily thin, since the parts
supporting the hinges need to be stiff and reﬂective, andbecause of its length, the extension beam is the softest.
However, a typical extension beam 5 mm thick is about
1000-fold stiffer than the hinges.
To fabricate membranes of controlled thickness, we haveused inexpensive and reproducible in situ depth rulers [38].
We typically produce membranes that are 10–20 mm thick,and thin them during tip production to 5–10 mm. After
further thinning occurring during the process of making
tips, the cantilever support beams and the pads are 5 mm
thick.
3.1.3. Integrated sharp tips
We use direct fabrication to integrate a variety of tips
shapes onto the probes. The process involves a controlled
undercut of a small masking island, which in our case is5  5 mm2. The tip is formed when the etchant undercuts
the island sufﬁciently to lift it off. We have three types ofmasks available—SiN, SiO2 and photoresist, so we can use
a variety of chemistries to produce tips. SiN is an excellent
mask for KOH, SiO2 is a very good mask for DRIE with

Fig. 5. Torsion levers for low-noise operation in liquid where viscous drag is high. (A) Symmetrically balanced torsion pad for low-force, contact mode,
imaging and force spectroscopy. (B) Asymmetric torsion pad for tapping mode.

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SF6 and photoresist may be used to mask SF6 and CF4
RIE. We have used SF6 to produce high-aspect atomically
sharp tips (Fig. 3A and B). The undercut of a SiN mask by
KOH produces octagonal tips with broad bases, four-foldsymmetry and atomically ﬂat walls (Fig. 3C and D). Tips
directly fabricated in KOH have low aspect ratio ¼ height/
width ¼ 1.2 and a cone angle of about 701. These tips are
not well suited for imaging high aspect ratio structures, butthey have well-deﬁned shapes and they can be used with
soft biological materials. We have been able to produce
taller tips by pre-etching a small post using DRIE prior torelease by SF6 and KOH [8,11,29]. Tips may be further
sharpened by oxidation [39,40] without additional processing if dry SiO2 is grown during the following steps.

allow the clean deﬁnition of two of the degrees of freedom
and simplify the deﬁnition of resonant properties.
3.2.1. Small, fast and soft torsion AFM cantilevers
The basic unit in our family of probes is the torsion levershown in Fig. 5. As described, the probe is small, compliant

3.1.4. Thin hinges
We deposit the hinge layer as a SiO2/SiN LPCVD stackwith the intent of using the SiN layer for the hinges and
SiO2 layer for protection. The thickness of the nitride layer
is critical in deﬁning the hinge stiffness. We haveexperimented with SiN thicknesses of 10–400 nm. A SiO2
thickness of 100 nm has proven adequate to protect thehinge during processing (Fig. 4).
Both the thin ﬁlms bear intrinsic mechanical stress(tensile for nitride and compressive for oxide) [13,41]. Dueto the symmetric design of the hinges, we have not seen the
stress-induced curvature that plagues asymmetrically supported cantilevers [14]. While the SiN/SiO2 hinges may be
used as is, we often soften them further with a short HF
etch to remove the SiO2. Since etch selectivity for oxideover nitride increases with decreasing HF concentrations[42], we use an aqueous mixture of 20% HF in 40% EtOH
and 40% H2O. Ethanol is added to the mixture to reducesurface tension. At this concentration of HF, the selectivity
for SiO2 to SiN is 100 to 1 [43], and the oxide layer is etchedwithin seconds, while the nitride layer is stable for minutes.
3.1.5. Metal coatings
We have deposited metal layers on the pad both toincrease reﬂectivity and to create magnetically sensitive
levers. To correct for the low reﬂectivity of silicon (30%)
to the red light [44] typical of lasers used in AFM, we havedeposited Au on the backside of the pad. A 50 nm Au layer
is sufﬁcient to achieve 90% reﬂectivity. While Au
coatings create a thermal bimorph which warps asymmetric cantilevers, our symmetric design with a rigid pad isimmune to this effect.
3.2. A family of torsion probes
The torsional arrangement is advantageous for several
reasons. During fabrication, the symmetry of the hinges
prevents warping. In operation, despite the ultra-thinhinges, the levers are stable during transfer across air/liquid interface. The small area of the pads minimizes drag,increases the frequency response and decreases the lowfrequency noise. Finally, orthogonally oriented hinges

Fig. 6. Dynamic properties of a typical symmetric soft torsion lever(20  20  5 mm3 pad on 10 mm long, 3 mm wide, 100-nm-thick hinges,
k ¼ 0:009 N=m). (A) Resonant frequency in air (f 0 ¼ 21 kHz, Q ¼ 53). (B)Resonant frequency in water (f 0 ¼ 12 kHz, Q ¼ 2:2). (C) Torsion levernoise in water in a 1 kHz band (F rms ¼ 0:82 pN).

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and retains a high resonant frequency in water (Fig. 6). The
levers are extended from the handle chip by a 150-mm-longextension beam. This distance is roughly the length of
traditional cantilevers and provides sufﬁcient clearance on
standard microscopes.
We sized the pad to accept the typical (20 mm) opticalbeam waist of commercial AFMs. Our process allows
further minimization, limited only by the optical resolutionof contact photolithography, 0.5 mm. An additional
factor in determining the pad length is the optical gain—

shorter pads have higher gains (yE1/l) [16]. Thus, shorterprobes may be made stiffer and still retain high sensitivity.
The hinges are the major determinant of mechanicalproperties of the lever. Using 50-nm-thick hinges, we canreliably build probes softer than the softest commerciallyavailable AFM cantilevers (0.01 N/m) and with resonances in water above 10 kHz (Fig. 6B).
These probes increase sensitivity, decrease noise andallow faster operation in all AFM operational modes [45].
Symmetric pads with minimum area and moment of inertia

Fig. 7. Magnetically driven pad. (A) Side view and (B) top view, 100 nm
cobalt layer deposited onto the topside of the torsion pad. (C) Actuationof the pad via a small non-calibrated electromagnet.

Fig. 8. Gimbaled frictional probes with two independent axes. (A) SEM
of a probe with soft SiN hinges. (B) Resonant peak in air (26 kHz) of theouter gimbal using the on B–T signal. (C) Resonant peak of the innerhinges (95 kHz) using the L–R signal.

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have the highest resonant frequencies and Q-factors. These
allow low noise recording (Fig. 6C) in open-loop force
spectroscopy, closed-loop contact mode and frequency
modulation imaging [46]. On the other hand, asymmetricpads (Fig. 5B) may be required to efﬁciently couple toexternal drive for tapping mode.
3.2.2. Magnetic drive
For dynamic studies using traditional methods, moving
the cantilever requires moving the entire cantilever support
structure with the scanning piezo [47] or moving thesubstrate [48]. This results in parasitic vibrations thatinterfere with the measurements. A much more ideal
solution would be move only the cantilever pad itself. A
ﬁrst approximation of this mode was done by Lindsay’s lab[32] and is currently available from Molecular Imaging. A
standard cantilever is coated with a magnetostrictive layer
and this magnetic bimorph is wiggled using an externalﬁeld (MAC mode AFM). However, as with all asymmetriclevers, inhomogeneities cause warping and drift. The
torsion lever, with its small rigid pad, is a more ideal
structure for magnetic drive. We can coat either thebackside or the topside of the pad with ferromagnetic
materials such as Co. On the backside, we deposit a
conformal layer, covering the pads as well as the hinges. On
the topside, we can control the geometry and deposit themagnet only onto the pad (Fig. 7A and B). If the coating isa hard magnetic material, it can be magnetized parallel tothe pad and torqued by a normal B ﬁeld [32] (Fig. 7C). A

soft magnetic coating can be torqued by a B ﬁeld at anangle. The magnetic coatings can be applied in vacuum
after fabrication with no additional masking necessary
since a small amount of metal on the torsion hinges does
not signiﬁcantly affect the stiffness. Calculations suggestthat with 100 nm coatings, 1 kG ﬁelds can apply 100 pN
forces to the tip. The magnetic drive is also well suited todrive tapping mode and FM modulation [46].
3.2.3. Dual axis (gimbaled) levers
We have made levers with two independent deﬂection
axes; a pad mounted on two sets of orthogonal springs
arranged as a gimbaled mount (Figs. 8A, 9B and C). These
levers provide several novel capabilities for the AFM:

 They can be used as exquisitely sensitive vectorial
friction probes, especially with tall tips, by locating the
tip where the hinge axes cross.
 With the tip located off the hinge centers, displacementof the two axes can be correlated to remove noise that
does not arise from z-axis displacements.
 Self-referencing levers that remove drift.
Excess noise in the AFM comes from instrumental drift
and environmental mechanical noise. If one could measure
the sample displacement relative to the substrate rather
than the laboratory reference frame, nearly all the excess
noise would be removed. Hoeber’s group [49] showed thatby using two separate levers, one touching the sample andFig. 9. Self-referencing AFM probes. (A) Cartoon of self-referencing probe. (B) Cartoon of probe used for self-referencing single molecule spectroscopy.
(C) Self-referencing AFM cell surface probe with a long reference beam for increased clearance of the sample tip from the substrate.

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one touching the substrate, they could easily achieve pN
force resolution. However, their technique is experimentally cumbersome, involving two separate optical levers ina custom AFM.
Our dual axis lever accomplishes the same task with asingle optical lever. The normal laser beam illuminates themirror, but the two orthogonal axes can be decoded by the
PDT (Bottom–Top, Left–Right). z-axis noise at higher
frequencies is also removed by taking the difference of thetwo signals (after appropriate scaling for differences in
optical gain of the two axes). Intensity ﬂuctuations of thelaser system can also be suppressed.
Fig. 8A shows a two-axis probe. Each of the vibration
modes shows up in the orthogonal direction on the PDT.
Flexing of the outer gimbal shows up at 26 kHz on thebottom–top (B–T) PDT channel (Fig. 8B). The inner pad
has a 95 kHz resonant peak on the left–right (L–R) channel(Fig. 8C). The resonant frequency and spring constant of
the inner pad may be engineered in the same way as for thetorsion probes that we discussed above. This probe may be
used for simultaneous friction and topography imaging.

845

Another application of this probe is for resolution of theangle between a tip-linked polymer and the z-axis of theAFM, a situation typical in dynamic force spectroscopy.
Off-normal angles cause underestimation of sample stiffness since only the z component is measured. With highcompliance springs and at small angles, the deﬂection angleof the pad will tend to become normal to axis of thepolymer. By using feedback to minimize the tipping angle,
the tension axis can be made normal to the substrate [50].
In another variation of the gimbaled probe, eachorthogonal axis has its own tip, one of which contacts
the sample and the other the substrate. Lever and tip
geometries can be varied depending upon the experiment(Fig. 9A). To examine the surface of cells, for example, thereference tip should be placed far from the sample tip toallow the sample tip clearance above the cell (Fig. 9C). For
increased detail resolution, the two tips can be placed close
together to create a differential contrast image of topology
or compliance (Fig. 9B). For single molecule force
spectroscopy, the dual axis lever provides not only
improved high-frequency response but also improved
low-frequency response by removing drift.
Our process allows simultaneous production of soft
hinges for the measurement levers and either soft SiN or
hard Si hinges for the reference levers. Soft reference hinges(Fig. 10A) maximize force sensitivity while stiff hinges (Fig.
10B) allow more stable reference contact and maximum
noise reduction.
4. Conclusion
We have developed a microfabrication process suitable formass production of torsion levers with a wide variety ofgeometries and mechanical properties. On a single die, we
can place multiple levers of the same or different size with awide variety of spring constants. These levers are asigniﬁcant advance in several areas: composite design, no
warping, reduced drag, increased frequency resolution, lower
noise and orthogonal sensitivity. The levers have moving
areas that need only be as big as the mirror required for the
optical lever. Minimal area results in optimal performance inliquids. While this paper has focused on applications of thetechnology to AFM cantilever design, the same fabrication
process can be applied to building sensors for multi-axis
accelerometers, gyroscopes, movable mirrors, switches,magnetometers, oscillators and other sensors.
Acknowledgments
Microfabrication was done at the Cornell NanoScale
Facility, Ithaca, New York. This work was supported by
NIH R01 GM060648.
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Fig. 10. SEM images of reference levers. (A) Soft reference lever. (B) Stiffreference lever on the same wafer using Si hinges.

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