Measuring and testing – Surface and cutting edge testing
Reexamination Certificate
2002-10-08
2004-01-13
Williams, Hezron (Department: 2856)
Measuring and testing
Surface and cutting edge testing
C073S082000
Reexamination Certificate
active
06675637
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a touch sensor for a probe, which, for example, is used when measuring micro-configuration of the surface of a test piece by equipment such as a micro-configuration measuring device and a surface roughness measuring device or inner configuration of a hole by a small hole measuring device.
2. Description of the Related Art
Conventionally, micro-configuration measuring devices are used when examining a test piece for research and development purposes as well as for production activities in the fields of precision machining or semiconductor manufacturing. The device measures micro-dimensions such as a surface roughness or a step on the machined surface and a thickness of a thin film by a vertically oscillating a stylus, which is brought in contact with and moved about the test piece. The change in the vertical oscillation of the stylus is then converted into an electrical signal to be read.
One example of such a mechanism involving a stylus used in micro-configuration measuring devices as described above is a touch sensor, which is disclosed in Japanese Patent Laid Open No. 2001-91206.
In
FIG. 6
, this touch sensor
10
includes a stylus holder
11
, a stylus
12
which is held by arms
13
,
14
,
15
and
16
and has a tip
12
A making contact with the test piece, and a couple of piezoelectric elements
19
, one of which is attached to the stylus on one side and the other of which on the side opposite thereto. Each piezoelectric element
19
is made of two parts, one being an oscillation means
17
and the other a detection means
18
, joining at the center.
Given such a structure, if an electrically alternating signal of an appropriate oscillation frequency is applied to the oscillation means
17
, then the stylus
12
starts oscillating in a resonating manner in the axial direction. If, in this resonating state, the tip
12
A of stylus
12
makes contact with the test piece, then the resonating state changes, and this change of state can be detected by monitoring output from the detection means
18
.
In precision measurement where micro-configuration is measured by using a touch sensor described above, it is important that measuring force acting between a test piece and the tip of a stylus be controlled below a prescribed value; the test piece and the tip not be damaged; and movement of the stylus tip accurately reflect the surface configuration of the test piece. Accordingly, a probe, which is equipped with a mechanism of controlling the measuring force below a prescribed value, is available.
One example is a probe for a micro-configuration measuring device disclosed in the U.S. patent application Ser. No. 09/805309.
In
FIG. 7
, this probe for micro-configuration measuring device is made of the above-mentioned touch sensor
10
and a fine motion mechanism
21
using a piezoelectric element (PZT), which are coupled together along the axis of oscillation of the stylus
12
and, as a whole, are attached to a movable support member
22
.
Given such a structure, if an electrically alternating signal, which is characterized by the oscillation frequency and the oscillation voltage, are sent from the oscillator
3
to the oscillation means
17
, then the stylus
12
starts oscillating in a resonating manner along its axis. If, in this state, the stylus tip
12
A makes contact with the test piece W, the resonating state of the stylus
12
changes. Accordingly, by monitoring output from the detection means
18
indicating this change, the contact between the stylus tip
12
A and the test piece W can be detected. Output from the detection means
18
, which is designated as the detection signal DS
1
, is sent out to a detecting circuit
4
. The detecting circuit
4
converts the detection signal DS
1
into the detection signal DS
2
. The detection signal DS
2
is filtered by a filter
51
to remove noises and sent out to a signal processing unit
62
as the detection signal DS
3
. The signal processing unit
62
computes a difference between the detection signal DS
3
and a threshold which determines the measuring force and sends out the result to a controller
61
. The controller
61
drives the fine motion mechanism
21
via a PZT driver
72
based on the result received. This system of controlling fine movement described so far allows the detection signal DS
3
to be maintained constant with respect to any bumps and dips on the test piece W when the fine motion mechanism
21
and the test piece W are in relative motion for scanning.
In order to be successful in making non-destructive measurement on a test piece such as a silicon wafer, it is important how much the measuring force can be minimized. And, in order to minimize the measuring force, it is necessary that sensitivity of a touch sensor be boosted or the threshold be raised. What was conventionally attempted for the minimization of the measuring force is the boosting of the sensitivity of touch sensors through modification of their structure. However, such modification was not able to produce satisfactory results regarding the performance of micro-configuration measuring devices.
SUMMARY OF THE INVENTION
A principal purpose of the present invention is to provide a touch sensor, which is capable of minimizing measuring force and making non-destructive measurement without damaging micro-configuration of the surface of a test piece.
Here, a relationship between the measuring force and the detection signal DS
3
in
FIG. 7
will be described.
FIG. 1
illustrates a setup for an experiment on static characteristics of the detection signal DS
3
during contact and non-contact conditions. A constant oscillation frequency is sent out from the oscillator
3
so that the stylus
12
oscillates in the direction of its axis. Then, a drive voltage from the PZT driver
7
is made to gradually increase so that the stylus
12
is brought closer to the test piece W. As the stylus
12
is further brought closer to the test piece W so that the tip
12
A of the stylus
12
starts making contact, force inflicted onto the tip
12
A gradually increases, and, at the same time, oscillation amplitude of the stylus tip
12
A gradually decreases.
FIG. 2
illustrates a relationship between the force inflicted on the stylus tip
12
A (the measuring force) and the detection signal DS
3
. The horizontal axis and the vertical axis of the graph represent the force inflicted on the stylus tip
12
A and the detection signal DS
3
, respectively. The detection signal DS
3
is maximum in the non-contact region. Assuming that where the detection signal DS
3
begins decreasing is the point of contact, the force inflicted on the stylus tip
12
A increases and the detection signal DS
3
decreases, both starting at the point of contact. A slope in this region is called a sensitivity gradient. The higher the sensitivity of the touch sensor is, the steeper the slope of the graph is. From the graph, it can be seen that, since the force on the stylus (the measuring force) is determined by the sensitivity gradient and the threshold, the minimization of the measuring force can be achieved either by boosting the sensitivity gradient or raising the threshold.
In conventional probes for micro-configuration measuring devices, the minimization of the measuring force was attempted by boosting the sensitivity of the touch sensor through modification of its structure. For example, in the touch sensor illustrated in
FIG. 6
, by designing flexural rigidity of the arms
13
,
14
,
15
and
16
to be lower than that of the stylus
12
in the axial direction, or by arranging the arms
13
,
14
,
15
and
16
symmetrically about the axis of the stylus
12
, flexural vibration of the stylus
12
with respect to its axis was prevented, thereby increasing the sensitivity of the touch sensor
10
. Since the sensitivity gradient becomes steeper for such design or arrangement, the force on the stylus tip
12
A (the measuring force) can be minimized, accordingly.
An inventor of the present invention has co
Mitutoyo Corporation
Rankin, Hill Porter & Clark LLP
Wilson Katina
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