Researchers designed and constructed a novel electron tunnel sensor which takes advantage of the mechanical properties of micro-machined silicon. For the first time, electrostatic forces are used to control the tunnel electrode separation, thereby avoiding the thermal drift and noise problems associated with piezoelectric actuators. The entire structure is composed of micro-machined silicon single crystals, including a folded cantilever spring and a tip. The application of this sensor to the development of a sensitive accelerometer is described

Kaiser, W. J.

Kenny, T. W.

Reynolds, J. K.

Waltman, S. B.

English

NASA Technical Reports Server

N91-2 ,9 067
Electron Tunnel Sensor Technology
T.W. Kenny, S.B. Waltman, J.K. Reynolds and W.J. Kaiser
Center for Space Microelectronics Technology
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
Abstract
We have designed and constructed a novel electron runnel sensor which takes
advantage of the mechanical properties of micro-machined silicon. For the first time,
electrostatic forces are used to control the tunnel electrode separation, thereby avoiding the
thermal drift and noise problems associated with piezoelectric actuators. The entire
structure is composed of micro-machined silicon single crystals, including a folded
cantilever spring and a tip. The application of this sensor to the development of a sensitive
accelerometer is described.
Devices used in the measurements of many physical quantifies (position, motion,
pressure, radiation, temperature) rely on conversion of the physical quantity to a change in
the relative separation between a pair of components. Changes in relative position are then
detected through changes in the capacitance between the two elements, 1-3 through optical
techniques,4, 5 or through flexing of piezoresistors. 6 In each case, large structures are
often required to achieve sensitivities that are not limited by readout noise in the transducer.
The availability of a more sensitive transducer would, in many cases, allow the
consu'uction of sensors with greater sensitivity, smaller mass, smaller volume, less power
consumption, or greater bandwidth.
Recently, electron tunneling through a narrow vacuum barrier has been employed
as a technique, Scanning Tunneling Microscopy (STM), for imaging the atomic scale
structure of surfaces. 7 The tunneling current, I, has the following dependence on the
separation between a pair of metallic electrodes, s :
I _ Vexp(-a'J-Os), (1)
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https://ntrs.nasa.gov/search.jsp?R=19910013754 2020-03-19T17:38:32+00:00Z
where • is the height of the tunneling barrier, V is the bias voltage, V is small compared to
_, and a = 1.025 (_,-I eV-I/2).7 For typical values of • and s, the current varies by an
order of magnitude for each 1 }_change in electrode separation. This sensitivity to relative
position is superior to that available in other compact transducers. Since tunneling only
occurs in regions where the tip is within several _ of the surface, the active area of the
sensor is microscopic. A position sensor based on electron tunneling has already been
incorporated into the design for an accelerometer, s and several other applications are being
considered.
Stable electron tunneling requires control of the separation of the tunnel electrodes
to a small fraction of an Angstrom. An analog feedback circuit is used to compare the
measured tunnel current to a set-point. The feedback circuit adjusts the voltage applied to
an electromechanical actuator to correct for discrepancies between the measured tunnel
current and the set-point. The bandwidth of the feedback circuit is limited to less than the
resonant frequency of the actuator in order to avoid instability.
Typical vacuum tunneling devices rely upon piezoelectric elec_mechanieal
actuators. These actuators suffer from sensitivity to thermal drifts, hysteresis,and creep in
the response of the piezoelectric materials; these effects impose severe limitations on the
performance of existing tunnel devices. Also, efforts to miniaturize sensors which
incorporate piezoelectrics are made difficult by the variety of different materials required for
the construction of the device.
There have been recent advances in the sophistication of micro-machining of silicon
through the use of anisotropic etchants as well as doping to control etching.9 These
advances have lead to the development of a new class of sensors composed entirely of
micro-machined silicon. There are several advantages to the development of sensor
components in silicon. First, the well-developed techniques of photo-lithography may be
used to control the structure of compact sensor components on the _.m-scale. Also, silicon
offers extremely high tensile strength combined with relatively low density. Further,
silicon single crystal structures show highly reproducible characteristics. Finally, micro-
machined silicon sensors are mass-produced inexpensively, and are easily miniaturized or
modified to meet the requirements of a particular application.
We have designed and constructed an electron tunnel sensor composed of micro-
machined silicon. Micro-machining has been used in this case to produce cm-scale
371
c,(m_ponentswith )_m-scale precision. In contrast to all previous vacuum tunneling devices,
the relative position of the electrodes is controlled through use of electrostatic forces applied
between the elements. The electrostatic forces induce deflection of a micro-machined
silicon cantilever spring. Replacement of the piezoelectric actuator with an electrostatic
actuator is important for the following reasons. First, the electrostatic actuator is
insensitive to thermal drifts and immune to the problems of creep associated with
piezoelectric actuators. Also the response of the electrostatic actuator is a function only of
the geometry and mechanical properties of the device, whereas the response of the
piezoelectric actuator is also dependent on the characteristics of the piezoelectric material,
which may not be reproducible between devices or over time. Finally, the electrostatic
actuator may be miniaturized more easily because the scaling laws are known exactly, and
the fabrication is less complex than for the piezoelectric actuator. The device described here
can be modified for incorporation into a wide variety of sensors. Monolithic devices
including sensor and control electronics are also feasible.
The tunnel sensor described here consists of three components which are
constructed from 200 pan silicon wafers. The wafers have been polished on both surfaces
and are coated with >0.5 )_mSiO2 which is patterned by standard photo-lithographic
techniques. The wafers are etched in Ethylene Diamine Pyrocatechol (EDP), removing the
parts of the silicon wafers not covered by the SiO2 mask. After etching, the remaining
oxide is removed in a buffered HF etch. A new oxide layer >1 _m thick is then grown on
all surfaces of the structure. Gold electrodes are thermally evaporated onto the components
the sensor through shadow masks which have been fabricated by the same micro-
machining techniques. The SiO2 serves as a dielecwic isolation layer between the metal
films and the silicon substrate. Figure 1 shows a sketch of the components which result
from etching and metallization. The components are approximately 4 crn2inarea. The
inner rectangular area of the folded cantilever spring can be deflected upward or downward
relative to the outer segments by application of an electric potential between the large
electrode and a corresponding deflection counter-electrode which is deposited on another
component of the sensor. Given the mechanical properties of the silicon as well as the
dimensions of the spring structure, we can calculate the properties of the spring. Because
the spring constant scales rapidly with the dimensions of the legs, its characteristics can
easily be tailored to meet the needs of a specific application.
372
Various methods for manufacturing a suitable tunneling tip are available.7, l0
Recently, we have developed the following procedure for micro-machining a silicon tip
directly from the silicon substrate. An oxide-coated surface of the silicon is t-u-st patterned
by photo-lithography to leave only a 60 gm x 60 I.tm square of oxide. When etched in
EDP, the edges of the oxide are undercut. When the undercutting is complete, the square
fragment of oxide is carried away, leaving a pyramid-shaped silicon tip. A scanning
electron microscope micrograph of a typical tip is shown in Fig. 1. The active surfaces of
the tips are prepared by evaporation of 3000 )k thick Au t'rims through a shadow mask.
The tunnel sensor is then assembled as shown in Fig. 2.
Once assembled, a bias voltage is applied to the electrostatic deflection electrodes to
close the electrodes and establish a tunnel current. Active regulation of the tip-electrode
separation using feedback control of the tunneling current is carried out as for STM. The
tunnel position sensor implemented in this way is generally applicable to a wide range of
sensing applications. The properties of a tunnel sensor applied to measurements of
acceleration are described below. Operation of the device as an accelerometer may be
achieved in either of two ways. In each case, acceleration is measured by sensing the
deflection of the spring-supported silicon mass.
In the first approach, denoted as open loop, acceleration is measured at frequencies
above the feedback loop bandwidth. When the sensor is subjected to an acceleration, 8a,
the deflection of the folded cantilever spring, _z, is given by _z = _a / (toO2 - 02) 11, where
= _ is the resonant frequency of the spring supported mass. 12 Since the current
varies as 31 = - I a _ _z, the responsivity is given by :
_I Ia ¢0 (2)
(000 2 - 0)2)
If the noise in the device is dominated by shot noise in the tunnel current, In = _ 2 e I ,
the noise-equivalent acceleration (NEa) is given by :
NEa = -_a = (_02-c°2)'_/2ea I_) (3)
The noise-equivalent acceleration is defined as the minimum acceleration that could be
detected in a 1 Hz bandwidth.
As shown in (2), the responsivity in the open loop method varies as _/-O, which
may depend on operating conditions. Variations in _ or other parameters which effect
373
the dependence of the tunnel current on electrode separation may be corrected for by adding
a small modulation to the actuator voltage and measuring the resulting modulation in the
tunnel current at that frequency. The ratio of the amplitudes of modulation directly yields
the sensor sensitivity. This simple in-situ calibration may be performed continuously
during operation of the sensor for applications which require high absolute accuracy.
In the second approach, denoted as closed loop, acceleration is measured for all
frequencies less than the feedback loop bandwidth. In this case, the feedback loop
responds to an acceleration by adjusting the force applied by the actuator so as to prevent
any changes in the relative displacement of the tunneling electrodes. In this approach, the
variations in the voltage applied to the actuator become the signal. For the electrostatic
actuator in the present device, the force required to maintain constant electrode separation is
given by:
e A V 2 (4)
F -- ma - 2d 2 ,
where A is the area of the deflection electrodes and d is the separation between the
electrodes. The responsivity is then given by
_V m d 2 (5)
eAV
An advantage of this closed-loop approach is that the responsivity (5) is now independent
of the height of the tunnel barrier or other properties of the tunnel electrodes as opposed to
case for the open-loop approach. Shot noise in the tunnel current at frequencies within the
bandwidth of the feedback loop causes fluctuations in the deflection voltage. These
variations in deflection voltage produce changes in tunnel electrode separation, and
therefore, variations in tunnel current which cancel the shot noise. The amplitude of the
shot noise-induced voltage fluctuations is given by :
2/'2-_k e A V
Vn _- N-_ _. m27_ (6)
The noise equivalent acceleration sensitivity is obtained by dividing the responsivity (5) by
the noise (6) and is exacdy the same as the open loop sensitivity (3).
The sources of noise observed in vacuum tunneling devices are not fully
understood. At high frequency, shot noise and Johnson noise are expected to dominate.
At lower frequencies, noise with a 1/f characteristic typically dominates. This noise may
originate from slow drifts in the device structure, or environmental sources of noise, such
as vibration, or air currents. Thermal drift, which leads to differential thermal expansion
may also contribute to the low frequency noise in vacuum tunneling devices. Other
374
mechanismsforI/fnoisehavebeenproposed.13,t4Withthereplacementofthe
piezoelectricactuatorby anelectrostaticctuator,theproblemsassociatedwithhysteresis,
creep and thermal drift in piezoelectric materials should be greatly reduced. Contributions
to the noise in tunneling due to fluctuations in the momentum u'anstx_ed have been
considered.15.16 This source of noise is not expected to be significant in our
measurements.
The ultinmte sensitivity of the tunnel sensor is limited by the noise characteristics of
the vacuum tunnel junction. With the feedback loop closed and constant electrode
separation being maintained, the tunnel current noise spectrum can be measured. Typical
spectra for a current of 1.3 nA and bias of 100 mV are shown in Fig. 3 for different
settings of the bandwidth of the feedback circuit. For this current, shot noise should
contribute 2 x 10-14 A/.FH-_,which is below the observed noise. The current noise due to
the components of the electrical feedback circuit is also well below the observed noise. The
tunnel current noise power spectrum varies approximately as 1If for frequencies below 103
Hz. This result is consistent with noise measurements reported elsewhere, 17and is likely
due to environmental noise sources such as vibrations or thermal drifts. At frequencies
greater than 104 I-Iz,an elecuical resonance related to the deflection electrodes introduces
additional noise.
By applying a low-frequency modulation to the tunnel current set-point in the
feedback circuit, a modulation in the deflection voltage and tunnel current are produced.
The barrier height, O, may be determined from the ratio of the amplitudes of the
modulations. For the Au electrodes in this device, measurements of O have given a value
of 0.5 eV. With • = 0.5 eV, the data in Fig. 4 implies that the measured sensitivity to
variations in the tip-sample separation is 3 x 10-4 _ at a frequency of 1 kHz. Given a
cantilever mass of 30 mg and a measured spring constant of 60 N/m as well as the
dependence of the tunnel current upon separation, the responsivity of the sensor to changes
in acceleration may be calculated. Below the resonant frequency of about 200 Hz, the
responsivity is 3 x 10-4/fig. With the observed noise of the sensor, this corresponds to a
sensitivity of 1 x 10-8 g/_ at 10 Hz. At 1 kHz, the sensitivity is 1 x 10.7 _. In
each case, the measured noise included contributions from accelerations in the laboratory,
so the actual sensitivity is probably somewhat better.
375
achieved. This would allow low-cost production of sensors of this type as transducers for
application to a wide variety of measurement needs. In particular, miniature pressure
sensors and microphones could be constructed with tunnel sensor technology. The use of
a tunnel sensor as the transducer in a pneumatic infrared detector is currently being
explored.l S
The research described in this paper was performed by the Center for Space
Microelectronics Technology, Jet Propulsion Laboratory, California Institute of
Technology, and was jointly sponsored by the Defense Advanced Research Projects
Agency and the National Aeronautics and Space Administration, Office of Aeronautics,
Exploration and Technology.
For comparison, the sensitivity of a capacitive displacement transducer may be
calculated. It is assumed that the electrode separation is no less than 1/100 of the electrode
length, that the voltage is no greater than the limit for dielectric breakdown in vacuum, and
that sensitivity is limited by the shot noise at the ac measurement frequency. With these
assumptions, the capacitive transducer has a theoretically-limited sensitivity to variations in
electrode separation of approximately 0.2 _/-_-H-_. In this limit, the theoretical capacitive
transducer is more than 3 orders of magnitude less sensitive than the existing tunneling
transducer, and more than 4 orders of magnitude less sensitive than the theoretical limit to
the tunneling transducer.
Since all of the components of the tunnel sensor can be prepared using standard
lithographic techniques, miniaturization over several orders of magnitude can be readily
376
1 F. Rudolf, Sensors and Actuators 4, 191 (1983).
2 K.E. Petersen, A. Shartel and N. Raley, IEEE Trans. Electron Devices ED-29, 23
(1982).
3 C.S. Sander, J.W. Knutti, and J.D. Meindl, IEEE Trans. Electron Devices ED-27, 927
(1980).
4 E. Stemme,and G. Stemme, IEEE Trans. Elect. Dev. 37, 648 (1990).
5 D.L. Gardner, T. Hofler, S.R. Baker, R.K. Yarber and S.L. Garrett, J. Light'wave
Tech. LT-5, 953 (1987).
6 H.V. Allen, S.C Terry and D.W. De Bruin, Sensors and Actuators 20, 153 (1989).
7 G. Binnig and H. Rohrer, IBM J. Res. Develop. 30, 355 (1986).
8 S.B. Waltman, W.J. Kaiser, Sensors and Actuators 19, 201 (1989).
9 K.E. Petersen, Proc. IEEE 70, 420 (1982).
10 R.B. Marcus et al. Appl. Phys. Lett. 56, 236 (1990).
11 In the low-frequency limit, this reduces to 0z = (m/k) 0a, which is Hooke's Law.
Damping has been ignored in this discussion.
12 It is assumed that the surface of the tunnel counter-electrode is flat and therefore ds =
dz. If the surface of the counter-electrode is rough, ds = dr/cos(Q), where Q is the angle
betweenthenormaltothesurfaceand thez axis.
13M.E. Welland and R.H. Koch. Appl.Phys. Lctt.48,724 (1986).
14J.B.Pcndry,P.D. Kirkman and E. Castano,Phys. Rev. Lett.57,2983 (1986).
15 M.F. Bocko, K.A. Stephcnsen,and R.H. Koch, Phys.Rcv. Lett.61,726 (1988).
16B. Yurkc and G.P.Kochanski,Phys.Rev. B tobc published(1990).
17D.W. Abraham, C.C. Williams,and H.K. Wickramasinghe, Appl.Phys.Lett.53,
1503 (1988).
18T.W. Kenny, S.B.Waltman, J.K.Reynolds,and W.J. Kaisertobe published.
FIGURE CAPTIONS
Figure 1.
Figure 2.
A drawing of the components of the tunnel sensor is shown. The folded cantilever
spring with integral tip is shown on the lower right. It is manufactured by patterning
both surfaces of the silicon wafer and etching until the tunnel tip had formed. The
components which support the tunneling counter-electrode and the deflection counter-
electrode are shown on the left and are manufactured by patterning only one surface of
the wafer. Au films are deposited through micro-machined shadow masks to form the
deflection and tunneling electrodes. The tabs with the square holes allow the
components to be constrained laterally by alignment pins.
A SEM micrograph of a typical micro-machined silicon tip. The tip has been coated by
3000 _, of Au.
Figure 3. An exploded view of the assembled tunnel sensor showing the orientations of the
components. From top to bottom are the deflection counter-electrode, the tunneling
counter-electrode, and the folded cantilever spring. The shaded regions on the spring
correspond to the Au films which are deposited through shadow masks.
Figure 4. This figure is a plot of the spectrum of the measured noise in the tunnel current at a
bias of 100 mV and current of 1.3 nA while operating in air. The bandwidth of the
feedback circuit was deliberately adjusted to be less than 0.1 Hz for the data shown as
circles. For the data plotted as squares, the bandwidth was set at the maximum value
that did not produce oscillations, which was 200 Hz for this device. A resonance in the
electrostatic deflection system contributes to the noise at frequencies above 5 kHz.
377
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Electron tunnel sensor technology

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