MASKLESS ELECTROCHEMICAL PATTERNING OF GOLD FILMS FOR
BIOSENSORS USING MICROMACHINED POLYIMIDE PROBES
Senol Mutlu, Amar S. Basu, Yogesh B. Gianchandani
Department of Electrical Engineering and Computer Science
University of Michigan, Ann Arbor, USAe-mail: smutlu@umich.edu, basua@umich.edu, yogesh@umich.edu
Abstract—This paper explores the electrochemical patterningof thin metal films using a scanning probe tip as an electrode.
In particular, a masking nickel film is etched to reveal a gold
pattern to which biomolecules like liposomes and proteins canbe attached after treatment with functionalized self assembled
monolayers (SAMs) of gold-thiolate.
The machining is
performed by ultracomplaint polyimide micromachinedprobes (with tip diameter <200 nm) as the machining tool in
NaCl:HCl electrolyte. These probes provide an embedded
electrical lead as well as high mechanical compliance thateliminates the need for servo control, which is typically needed
to maintain the proximity between the workpiece and the tool.
By applying 500 ns, 2 V pulses, 3 µm wide patterns were
fabricated. Experiments suggest that they can be scaled down
to tens of nanometers by using 5 ns pulses. Selective
adsorption of undecanethiol SAMs to the patterned gold wasconfirmed using XPS spectroscopy.

I.

INTRODUCTION

For rapid prototyping and low volume applications,electrochemical micromachining (ECM) can provide costand speed advantages over lithography-based methods of
selective deposition or removal. Maskless patterning can beachieved with ECM by locally and selectively etching metals
with a scanning electrode tip. This kind of etching not only
creates very precise patterns, but also reduces chemical
wastes and effluents [1]. Furthermore, with the rightconditions, the machining tool does not get degraded during
etching. The use of ECM to fabricate microstructures dates
back at least to 1955, when it was used for micromachining
germanium and silicon with minimal debris [2]. Morerecently, the precision of ECM has been shown bymachining nano patterns on bulk metals like stainless steel
and copper using the nano tips of sharpened metal electrodes
and applying nano second pulses of voltage signals between
the workpiece and the tool separated by a servo-controlled
micron-sized gap [3, 4].
In this effort, a micromachined polyimide probe with ametal coated tip of diameter <200 nm and embedded
electrical leads [5, 6] is used as a scanning electrode for
ECM. Using short electrical pulses, the probe patterns athin-film Ni layer that covers a layer of Au. The low
mechanical stiffness of the probe (typically ~0.1 N/m)eliminates the need for servo control to maintain proximity
between the workpiece and the tool, further simplifying the

Fig. 1. Working principle of the electrochemical etching with amicromachined probe (top). Biosensor application of a patterned gold
surface by electrochemical etching of nickel mask on top.

fabrication process. In this approach, probes can touch thesubstrate without causing an ohmic contact. The low spring
constant ensures that the contact pressure of the probes
remains low and does not vary significantly as they arerastered over minor topological variations.
As one possible application, this method allows
inexpensive non-lithographic patterning of functionalizedchemicals for chemical and biosensors. After gold patterns
are revealed, the surface can be treated with functionalized
thiolates to form self-assembled monolayers (SAMs).
Various biomolecules such as liposomes [7] and proteins [8]can be selectively attached to the SAMs and used inbiosensor applications as depicted in Fig. 1. SAMs havebeen routinely used for micro-patterning proteins and cells
for biosensors and fundamental studies in cell growth andproliferation. They have also been used in chemical sensors[9].
Past approaches to forming SAMs on gold substrates
include micro-contact ‘inking’ with an elastomeric stamp, as
well as conventional lithographic patterning of gold [7].
Gold is a preferred substrate for adsorbing alkanethiol SAMs
because it reacts strongly with sulfur atoms, and it is resistant
to atmospheric contamination. Oxidizable metals such as Ni,
on the other hand, tend to resist the formation of SAMs
because the metal oxides hinder the formation of a stablechemical bond between sulfur and the metallic atom.
Additionally, they tend to oxidize thiolates into sulfinatesand sulfonates which do not bind strongly to the surface.
Unless it undergoes an electrochemical reduction
pretreatment [10], Ni can act as a mask to block SAM
formation.
II.

ELECTROCHEMICAL ETCHING

In an electrolyte, the charge transport is achieved by the
motion of ions whereas in the electrode it is by the electrons.
Electrochemical cells use at least two electrodes separated byat least one electrolyte phase. The overall chemical reactionin the cell consists of two half-reactions, each at one
electrode. In this work, a gold electrode on the surface of theprobe tip was used as a machining tool, a nickel surface usedas the workpiece, and NaCl/HCl solution used as theelectrolyte form the electrochemical cell. A very shortsquare voltage pulse (around 500 ns) with a small duty cycle(usually ~10%) was applied between the workpiece (anode)and the tool (cathode) while the probe, mounted on an XYZ
stage, was moved with a constant feedrate, selectively
etching nickel in the tip area (Fig. 1). The z-axis control ofthe stage was only used to initially touch the probe tip to thesubstrate. However, this does not cause an ohmic contact
between the probe tip and the substrate because of the low
contact pressure permitted by the mechanical compliance ofthe probe shank.
With the application of short duration voltage pulses, twokinds of processes occur at the interface between theelectrode and the electrolyte: non-faradaic and faradaic
processes [11]. In the non-faradaic process, a capacitive
action such as charging of the double layer, takes place.
Most solid surfaces, including metals, acquire a finite charge
density at the interface upon contact with an aqueous
solution. The charged surface attracts counter-ions andrepels co-ions in the solution forming a double layer of ions,consisting of a compact immobile layer and a mobile diffuse
layer in the liquid. Some of the electrons from the applied
AC voltage source are used to charge and discharge thisdouble layer. In parallel to this, faradic processes (chemical
reactions) take place involving the exchange of electrons
across electrode-electrolyte interface causing electrochemical
etching according to the Faraday Law:

Rate (mols 1cm  2 ) =

if
zFA

(1)

where Rate is the dissolution (etching) rate of the material inmole per second per square centimeter, if is the faradaic
current flowing through the electrode with a cross-sectional
area of A, z is the number of electrodes involved in thereaction and F is Faraday’s constant. The Faradaic processthat is utilized in this work is the dissolution of nickel on theworkpiece and the evolution of hydrogen gas at the gold tool
piece as summarized below.
Cathode: 2H2O + 2e-  H2(g) + 2OHAnode: Ni(s) + 2Cl-  NiCl2 + 2e-

Fig. 2. Equivalent circuit of the electrochemical cell for small electrodeelectrolyte interface potentials.

An electrochemical reaction does not start unless theinterface potential, VInt, is greater than the equilibrium
potential, VEq, of the reaction. When the interface potential
exceeds this, the reaction at the electrode-electrolyte
interface is proportional to the current passing through theinterface and this current is exponentially dependent on thepotential difference, (VInt – VEq), which is referred to as
overpotential. The relation between the current and theoverpotential across the interface can be approximated by thefollowing equation, which is a simplified Butler-Volmer
equation [11]:i f = i0 e

 (Vint VEq ) / zVt(2)

where i0 is the exchange current density,  is the transfer
coefficient, which is usually 0.5, and Vt equals to 25 mV.
This process can be modeled by a diode. However, eventhough this relationship at the interface is highly nonlinear,for simplicity, it is usually represented by a charge-transfer
resistance, Rct, which is a linearized value for the exponential
relation for a small potential difference across the interface.
This process is said to operate in the charge-transfer region,
meaning that the reaction is limited by the time it takes totransfer charges across the interface.
The overall
electrochemical cell can be represented as shown in Fig. 2,
with bulk of the solution represented as an ideal resistor, Rel.
Cpar represents the parasitic capacitance between theinsulated metal lines on the polyimide probe shank and thebulk solution. Rpar represents the parasitic resistance fromthe power source to the tip of the probe.
For larger interface potentials, the electrochemical
process becomes diffusion limited and a maximum current
level is reached because now all the reactants are depleted
around the electrode interface and have to come from thebulk through mass transport, i.e. diffusion. Because of this,the reaction is said to be diffusion limited and the system
becomes even more nonlinear [12]. In this work it isbelieved that the applied potential does not cause the system
to go into the diffusion limited case, but stays within chargetransfer limited region.
Short voltage pulses in the nano second range charge
double layers, increasing the potential across them. This
increased potential causes a faradaic current flow at theinterface, in turn causing an electrochemical etch. Because
this current is exponential with the developing potential
across the interface as shown in (2), and because the etch rate

Fig. 3. Schematic of the micromachined probe (from [5]).

is proportional to the current passing as shown in (1), theetching is confined to the area very close to the probe tip. As
a result, the etch rate and confinement of the etch depend on
the amplitude of the applied voltage, its duration, the gap
between the probe and the workpiece and the conductivity ofthe electrolyte.
III.

MICROMACHINED POLYIMIDE PROBES

In electrochemical etching, the gap between theworkpiece and the machining probe tip determines the
distribution of current flux, and hence the spatialconfinement of the etch. It also determines the solution
resistance between the tip and the sample. Because of this,the gap is kept around one micron with closed loop control[3, 4]. This requires a circuit to sense the gap and acomputer interface to control it dynamically. However, in
this work, the micromachined ultracompliant polyimide
probe eliminates the need for servo control. The contact
force between the tip and the sample is less than 300 nN fora probe shank deflection of 1 µm, which is too low to form
an ohmic contact. Consequently, the probe can be rasteredacross the substrate to make patterns while accommodating
significant topographical variations [5].
The fabrication details and some other applications of
this probe are provided in [5] and references therein. The
final structure is a cantilever (typically 360 µm long and 120µm wide) in which two layers of polyimide sandwich two
250 nm-thick Au leads. At the distal end, this layer is
exposed and extended over a protruding pyramidal tip. The
entire structure is attached to an insulated Si substrate on
which metal pads provide electrical access to the embedded
leads. A schematic and an SEM picture of the fabricatedprobe can be seen in Figs. 3 and 4, respectively. The probe
die is attached to a PCB and wire bonded. As a final step,
PCB, wires and electrode pads of the probe is painted with
polyvinyl butyral (Wacker Chemicals) dissolved in IPA.
After IPA evaporates, the structures are coated with a thininsulating polymer layer.
IV.

Fig. 4. SEM picture of a
micromachined probe (from [6]).
The flipped probe shank extends
beyond the edge of the substrate.
Inset shows the scanning tip.

describe earlier. Pulses of 500 ns width, 22.8 µs period, anddifferent amplitudes were applied between the probe thesubstrate. The resulting patterns for 2 and 3 V amplitudes
are shown in Fig. 5. Figure 6 shows the current response toan applied voltages pulse. The current flow is typical of an
RC circuit, indicating the absence of a short circuit between
the tip and the sample.
The dependence of etched pattern widths on applied
voltage amplitude is shown in Fig. 7. As expected from (2),the etch rate increases exponentially with the developing
potential at the interface; therefore, the pattern width has an
exponential dependence on the amplitude of the applied
voltage pulse. Also shown in Fig. 7 is the impact of varying
the pulse duration. Here, electrochemical etching with afixed 3 V amplitude, but different pulse durations(corresponding to a change in duty cycle), was performed ina 0.2 M NaCl and 0.1 M HCl solution. Assuming that theinterface potential is due to charging of an RC circuit with a
specific time constant, the developing potential depends
exponentially on the duration of the applied pulse. From (2),the etch rate also depends exponentially on the pulse
duration in agreement with the etching results in Fig. 7.
A separate experiment was conducted to verify theformation of SAMs on the electrochemically exposed gold
surfaces. Here, a 1.2 µm diameter tungsten microprobe tip
was insulated with a thin layer of polyvinyl butyral
everywhere except the tip region, which was left uncoated.
Several large (>500 µm) gold patterns were formed byelectrochemically etching the nickel mask with 2V, 500 ns
pulses in a 1M NaCl and 0.5 M HCl solution. After rinsing
the sample in DI water and IPA, it was immersed in a 1 mM
ethanol solution of undecanethiol (Aldrich) for 6 h to form ahydrophobic alkanethiol SAM. The samples were thenrinsed in ethanol to remove excess SAM molecules.

EXPERIMENTAL RESULTS

Electrochemical
etchingofevaporated
nickel(800/400/200 nm Ti/Au/Ni) on a silicon substrate was
performed using the setup shown in Fig. 1. The electrolyte
was composed of 1 M NaCl and 0.5 M HCl solution,corresponding to a pH of 0.3. In all experiments, the probe
tip was in mechanical contact with the substrate but there
was no ohmic contact because of the low contact force as

Fig. 5. Optical images of patterned lines using the probe in a 1 M NaCl and
0.5 M HCl solution with a feed rate of 20 µm/s. Left: 500 ns pulses with a
22.8 µs period and 2 V amplitude. Right: 500 ns pulses with a 22.8 µs
period and 3 V amplitude.

Fig. 6. Applied 10 V voltage pulse of 500 ns duration with a 22.8 µs period(top) in 0.5 M NaCl and 0.5 M HCl solution and the current response of theelectrochemical cell (bottom). Close up view on the right.

The formation of the SAM was verified by X-ray
photoelectron spectroscopy (Al-K source, 1486.6 eV).
XPS spectra in the S(2p) region were obtained on both theexposed gold and the unetched nickel regions. Figure 8
shows that the thiolates selectively adsorb to the etched gold
regions but not the nickel mask. The sulfur doublet observedin the gold spectrum consists of two peaks, S(2p1/2) at 163.3
eV and S(2p3/2) at 161.8 eV. The 1:2 ratio of the respective
peaks, commonly observed in gold-thiolate SAMs,
represents a shift towards lower binding energies andindicates that the free thiols are bound to the metallic surface[10]. No thiolates are detected in the nickel spectrum.
V.

CONCLUSIONSIn this work, selective electrochemical etching of nickel
films on gold was achieved by applying nanosecond voltage
pulses between the nickel film and a micromachined
ultracompliant polyimide probe.
Application of shortvoltage pulses confined the etch area to the probe tip area bycausing an exponential dependence between the distance andthe etch rate. The mechanical compliance of the probe shank
to topographical variations eliminated the need for servo
control to maintain the gap between the probe tip and thesubstrate in the one-micron regime for precise machining.
By applying 500 ns, 2 V pulses, 3 µm wide patterns wereachieved in a 1 M NaCl and 0.5 M HCl solution.
Experiments suggested that they could be scaled down totens of nanometer by using 5 ns pulses. Selective adsorptionof alkanethiol SAMs to the Au regions was verified using
XPS, and Ni was shown to be an effective mask.

Fig. 7.
Control of patterned line widths. (left) Line widths versus thepulse amplitude while keeping pulse duration constant at 500 ns in a 1 M
NaCl and 0.5 M HCl solution. (right) Line widths versus pulse duration
while keeping a fixed 3 V amplitude in a 0.2 M NaCl and 0.1 M HCl
solution. In both experiments, the pulse period and feed rate are 22.8 µs
and 20 µm/s, respectively.

Fig. 8. XPS spectra (S2p region) of the electrochemically patterned gold(top) and the unetched nickel mask (bottom) after immersion in an ethanol
solution of undecanethiol. Thiolates selectively adsorb to the gold regions.
ACKNOWLEDGEMENT
Facilities used for this research include the Michigan Nanofabrication
Facility (MNF) and the Electron Microbeam Analysis Laboratory (EMAL)
at the University of Michigan. The authors would like to thank Dr. Shamus
McNamara for the fabrication of the probes, and Dr. Kai Sun for his
assistance with XPS spectroscopy. This work was supported in part by theEngineering Research Centers Program of the National Science Foundation
under Award Number EEC-9986866.
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