Download a nanometer profiling device with contact force

A NANOMETER PROFILING DEVICE WITH CONTACT FORCE
CONTROLLED AT THE LEVEL OF MILLINEWTONS
L. Yang *, Y. Gao **, and T. Xie *
* Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China
** Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
E-mail: [email protected]
Introduction
It is known that in order to measure the surface profiles or the 3D topography, contact or noncontact type profiling devices can be considered. The non-contact type instruments are typically
optical ones and rely on the reflected lights from the measured surfaces. Due to the lights being
used, the non-contact profile devices are advantageous as no force is involved in the
measurement. For many applications, in particular, for the micro-system applications where the
steep steps and transparent thin films are involved, the non-contact profiling devices may not
work effectively. In these cases, the contact profiling devices have to be utilized. Contact
profiling devices utilize a contact profiling probe or a stylus to make contact with the surfaces
under measurement. Inevitably, a contact force is induced. Due to the contact force, scratches
may be produced on the surface and the measurement results distorted. It is clear that the contact
force should be maintained as small as possible. A number of research efforts have been aimed
to deliver improved stylus designs to reduce the contact force. However, due to the feature of the
surfaces under measurement, the contact force would vary in a measurement process. This would
make the measurement results very unstable and the uncertainty deteriorated.
In order to have a high precision and a large measurement range, some research has been
conducted and new probes or devices have been developed. A stepper motor [1] could be used to
extend measurement range. When measuring signal exceeds measurement range, the probe
moves up or down by driving motor. As a result, measuring signal changes smaller and return
into the measuring range, the measuring range is enlarged. The disadvantage of this method is
lower system accuracy due to lower moving precision and control precision, inconvenience of
control of measuring force. Piezoelectric ceramics, glass ceramics with super-lower inflation
coefficient, and capacity sensors could be used [2-4]. It adopts PI reactive control circuit to
adjust measuring force. This method has higher precision, and the probe can maintain a constant
tip-to-sample force. The measuring force was at 100nN approximately. The disadvantages of the
design of the type were high cost, difficult to manufacture, and small measurement range. In
order to solve the problem, a new profiling device with constant contact force is proposed and
developed. In this device, the contact force is controlled using an inductive probe of high
resolution and a voice coil servo system to reduce and to control the contact force to the level of
millinewtons. The voice coil servo system and the probe are built in the device to be an
integrated part of the profiling device to ensure consistent performance in a profiling process.
Working Principle
The structure of the proposed device is shown in Fig. 1. It consists of a voice coil motor (VCM)
with a short and movable coil and an electric inductance sensor. As an executive mechanism, the
VCM consists of a magnet core, a coil, a moving part, and springs. The VCM avoids the wear in
a driving mechanism, has low noise, a simple structure, a high working frequency. Compared to
other VCMs, the motor with short and movable coil has advantages of good dynamic response
due to lighter mass of moving part and less inductance due to short moving coil. This is good to
improve the dynamic stability of the control system and simplify the design of compensatory
network of phase. In the profiling device, the VCM was used as a part of the servo control
system used to enlarge the measurement range.
1 - installation rack 2 - framework 3 - coil of induction 4 - induction magnetic core
5 - stylus
6 - reed
7 - holding rack
8 - iron post
9 - shell
10 - alnico
11 - coil
12 - moving part
13 - nut
14 - reed
Fig. 1 Structure of the profiling device
The stylus is connected to the magnet core of the electric inductance through two reeds installed
on holding rack. When the stylus moving through the measured surface, at the same time,
through two reeds installed on holding rack, stylus pulls magnet of inductive sensor to move,
displacement signal is generated. This signal is transmitted to processing circuit of electric
inductance. In order to improve measurement accuracy, electric inductance should be high
precise and should work in very little and linear range. Usually, the measurement range of
electric induction should be smaller than 3µm according to the precision profiling device
required. The VCM consists of coil, magnet, alnico, iron post, moving part, etc. The coil of this
motor used in the profiling device is movable and short. Electric induction sensor is installed at
the bottom of the framework of the VCM.
In order to extend the measurement range of the device, the holding rack and the electric
induction, connected with holding rack, are connected with the moving part of VCM. The
moving part of the motor is connected with two reeds installed on installation rack. In addition to
the structure as the above, other types of structures can be used. For example, the VCM can have
a long coil. Other sensors, such as a differential transformer or a capacity displacement sensor,
could be used, although the precision may be affected. No matter which structure used, the
moving part of VCM must be connected with moving part of electric induction or differential
transformer. The sensor used in this profiling device must have high precision, little measuring
range as small as possible, fine linearity. The enlargement of measuring range must be conducted
by driving moving part of VCM up or down. In addition, the electric induction or capacity
displacement sensor must be working in very little measuring range in order to improve
resolution of profiling device. The holding rack and electric induction sensor are connected with
moving part of VCM. When voltages are applied to the coil of the motor, the moving part will be
located at corresponding position due to magnetic force. As a result, the force between the stylus
and workpiece will vary. When the moving part moves, it drives the holding rack up and down.
Therefore, the electric induction sensor would be moved up or down.
Signal processing
Signals of the electric induction are transmitted to a computer after they are processed by the
signal processing circuit, and converted by the A/D converting circuit. The signal processing
circuit is required for high stability, lower electric noise, and high resolution. When the magnet
of electric induction located at the predetermined limit position, the voltage input of signal
processing circuit is also a predetermined voltage, such as 1-4V. The output of A/D converter is
also a predetermined value near to LSB and MSB of A/D converter. When measuring a
workpiece, if displacement varies in measuring range of electric induction, the moving part of
the motor will not be adjusted. When the output of A/D converter exceeds predetermined value,
the voltage applied to coil of VCR will be adjusted through D/A converting circuit. As a result,
the moving part will be driven up or down. Making use of a digital PID control algorithm, the
adjustment value of the moving part is determined by the computer, according to the voltage
applied to the coil of the VCM.
Therefore, the electric induction will work in the pre-determined measurement range. The
measurement force will be adjusted. Because the measuring force of the profiling device can be
known by a computer from the value of A/D converter and the moving part of the motor may be
adjusted, the measuring force between stylus and surface of workpiece can be adjusted to the
level of millinewtorns, due to using the PID feedback algorithm, and can be maintained almost at
a constant value. Therefore, the stylus would not damage the surface of a workpiece. There are a
number of error sources in this profiling device. The friction on the magnet, the damping of the
reeds, the fluctuation of voltage, magnetic lag of the VCM, resolution and instability of the
electric induction sensor are the typical error sources. The magnetic lag is an important factor.
There is a hysteresis error between the moving parts of VCM moving up and down. In order to
solve the problem, the VCM position must be calibrated with high precision.
Applications
The proposed profiling device has been used in a measurement system to trace marks on bullets
for forensic investigations (Fig. 2). Typical measurement results are as shown in Fig. 3. The
application required a measurement range at least at 200µm. In addition, good longitudinal and
lateral resolutions were required. In the measurement system, the probe responsible for the
contact force control was set to a 10µm range to ensure a sensitivity level necessary for the
application.
Due to the use of a high stability signal processing circuit, the signal of the probe output could be
controlled within 1mV. The probe output was used as feedback signal to control the voice coil
servo system to reduce the measurement force. As a result, the indention of the probe was
controlled to be less than 5µm, and the measurement force was estimated to be at the 0.0lmN
level, at least at the level of millinewtons. In a series of experiments, measurement marks could
not be traced using a 40x optical microscope. The resolution of the profiling device was 10nm,
and the measurement range was up to 2mm. The proposed profiling system can be a good
supplement for high precision, low cost profiling applications.
Fig. 2 A system utilizing the profiling device
Fig. 3 Measurement results of the system
Conclusions
A profiling device with a resolution of 10nm and a measurement range up to 2mm has been
developed. The device should have a precision better than 0.1µm. It has been used successfully
in a demanding measurement application. In this device, an inductive probe of high resolution
and a voice coil servo system were used to achieve millinewton level measurement forces. The
device has a simpler structure and is easy to use and manufacture.
References
[1] Chuard, M., Mignot, J., Nardin, P., and Rondot, D., 1986, Range expansion and automation
of a classical profilometer, Journal of Manufacturing Systems, 6(3), pp. 223-230.
[2] Howard, L. P., and Smith, S. T., 1992, Long range constant force profiling for measurement
of engineering surfaces, Review of Science Instrument, 63(10), pp. 4289-4295.
[3] Chetwynd, D. G., Liu, X., and Smith, S. T., 1996, A controlled-force stylus displacement
probe, Precision Engineering, 19(2-3), pp. 105-111.
[4] Howard, L. P., and Smith, S. T., 1994, A metrological constant force stylus profiler, Review
of Science Instrument, 65(10), pp. 892-902.