Measuring and testing – Surface and cutting edge testing – Roughness
Reexamination Certificate
2001-03-09
2003-09-02
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Surface and cutting edge testing
Roughness
C250S306000, C250S227260, C250S234000
Reexamination Certificate
active
06612160
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to scanning probe microscopes (SPMs) and other related metrology apparatus. More particularly, it is directed to apparatus and methods for measuring the movement of a probe of, for example, an SPM, and minimizing the negative effects associated with parasitic movement of such a probe.
2. Discussion of the Prior Art
Scanning probe microscopes are typically used to determine the surface characteristics of a sample, commonly biological or semiconductor samples, to a high degree of accuracy, down to the Ångstrom scale. Two common forms of the scanning probe microscope are shown in
FIGS. 1A and 1B
. A scanning probe microscope operates by scanning a measuring probe assembly having a sharp stylus over a sample surface while measuring one or more properties of the surface. The examples shown in
FIGS. 1A and 1B
are atomic force microscopes (“AFMs”) where a measuring probe assembly
12
includes a sharp stylus
14
attached to a flexible cantilever
16
. Commonly, an actuator such as a piezoelectric tube (often referred to hereinafter as a “piezo tube”) is used to generate relative motion between the measuring probe
12
and the sample surface. A piezoelectric tube is a device that moves in one or more directions when voltages are applied to electrodes disposed inside and outside the tube (
29
in FIG.
1
C).
In
FIG. 1A
, measuring probe assembly
12
is attached to a piezoelectric tube actuator
18
so that the probe may be scanned over a sample
20
fixed to a support
22
.
FIG. 1B
shows an alternative embodiment where the probe assembly
12
is held in place and the sample
20
, which is coupled to a piezoelectric tube actuator
24
, is scanned under it. In both AFM examples in
FIGS. 1A and 1B
, the deflection of the cantilever
16
is measured by reflecting a laser beam
26
off the back side of cantilever
16
and towards a position sensitive detector
28
.
One of the continuing concerns with these devices is how to improve their accuracy. Since these microscopes often measure surface characteristics on the order of Ångstroms, positioning the sample and probe with respect to each other is critical. Referring to
FIG. 1C
, as implemented in the arrangement of
FIG. 1A
, when an appropriate voltage (V
x
or V
y
) is applied to electrodes
29
disposed on the upper portion
30
of piezoelectric tube actuator
18
, called an X and Y-axis translating section or more commonly an “X-Y tube,” the upper portion may bend in two axes, the X and Y-axes as shown. When a voltage (V
z
) is applied across electrodes
29
in the lower portion
32
of tube
18
, called a Z-axis translating section or more commonly a “Z-tube,” the lower portion extends or retracts, generally vertically. In this manner, portions
30
,
32
and the probe (or sample) can be steered left or right, forward or backward and up and down. This arrangement provides three degrees of freedom of motion. For the arrangement illustrated in
FIG. 1A
, with one end fixed to a microscope frame (for example,
34
in FIG.
1
D), the free end of tube
18
can be moved in three orthogonal directions with relation to the sample
20
.
Unfortunately, piezoelectric tubes and other types of actuators are imperfect. For example, the piezo tube often does not move only in the intended direction.
FIG. 1D
shows an undesirable, yet common, case where a piezo tube actuator
18
was commanded to move in the Z-direction (by the application of an appropriate voltage between the inner and outer electrodes,
29
in FIG.
1
C), but where, in response, the Z-tube
18
moves not only in the Z-direction, but in the X and/or Y-directions as well. This unwanted parasitic motion, shown in
FIG. 1D
as &Dgr;X, limits the accuracy of measurements obtained by scanning probe microscopes. Similar parasitic motion in the Y-direction is also common. The amount of this parasitic motion varies with the geometry of the tube and with the uniformity of the tube material, but typically cannot be eliminated to the accuracy required by present instruments.
Current methods of monitoring the motion of the probe or sample
20
when driven by a piezoelectric tube are not sufficiently developed to compensate for this parasitic X and Y-error. The devices are typically calibrated by applying a voltage to the X-Y tube and the Z-tube, and then measuring the actual distance that the probe travels. Thus, the position of the free end of the piezo tube is estimated by the voltage that is applied to the X-Y tube and the Z-tube. However, because the (X,Y) position error introduced by the Z-tube on the probe (or on the sample for the arrangement shown in
FIG. 1B
) is essentially random, it cannot be eliminated merely by measuring the voltage applied to the Z-tube or to the X-Y tube.
Moreover, with respect to movement in the intended direction, piezoelectric tubes and other types of actuators typically do not move in a predictable way when known voltages are applied. The ideal behavior would be that the actuator move in exact proportion to the voltage applied. Instead actuators, including piezo tubes, move in a non-linear manner, meaning that their sensitivity (e.g., nanometers of motion per applied voltage) can vary as the voltage increases. In addition, they suffer from hysteresis effects. Most generally, the response to an incremental voltage change will depend on the history of previous voltages applied to the actuator. This hysteresis effect, thus, can cause a large prior motion to affect the response of a commanded move, even many minutes later.
Notably, also, vertical measurements in scanning probe microscopy are typically made by moving the probe up or down in response to the rising or falling sample surface. For example, for AFM operation in tapping mode, the actual vertical measurement is the average distance the probe moves in the vertical direction to maintain a constant oscillation magnitude as it taps the surface, while for AFM operation in contact mode, the vertical measurement is the distance the probe moves to maintain a particular amount of force between the cantilever stylus and the sample surface. This distance is often calculated mathematically by recording the voltage applied to the piezoelectric tube and then multiplying by the tube's calibrated sensitivity in nm/V. But as mentioned previously, this sensitivity is not constant and depends on the previous voltages applied to the tube. So using the voltage applied to the tube to calculate the vertical motion of the tube will always result in an error with respect to the actual motion. This error can translate directly into errors when measuring surface topography of a sample.
What is needed, therefore, is an apparatus and method for controlling the motion of the probe or sample to minimize the effects due to, for example, adverse parasitic motion introduced by an actuator (e.g., a Z-tube) in a metrology apparatus. Moreover, an apparatus is needed to measure the magnitude of the intended motion to, for example, track whether intended motion is being realized.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for controlling the motion of a metrology probe to minimize negative effects associated with parasitic motion introduced, for example, by an actuator (e.g., a Z-tube) in a metrology apparatus such as an SPM or a profiler. In addition, the apparatus measures the magnitude of the actual motion. Notably, the apparatus implements an optical detection apparatus, utilizing an arrangement of reflecting surfaces disposed in corner-cube retroreflector-like relationship, so that the light reflected and sensed by the apparatus is relatively immune to, for example, lateral deflections of components of the microscope coupled thereto.
According to first aspect of the preferred embodiment, a Z isolating/measuring assembly for a metrology apparatus includes a piezoelectric or electrostrictive actuator assembly including a first actuator stage configured to controllably move in first and second o
Massie James R
Proksch Roger B
Boyle Fredrickson Newholm Stein & Gratz S.C.
Larkin Daniel S.
Veeco Instruments Inc.
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