Interpolated height determination in an atomic force microscope

Details Interpolated height determination in an atomic force microscope Interpolated height determination in an atomic force microscope

1999-12-16

2001-06-12

Williams, Hezron (Department: 2856)

Measuring and testing

Surface and cutting edge testing

Roughness

C250S306000, C250S307000

Reexamination Certificate

active

06244103

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to profilometers or atomic force microscopes. In particular, the invention relates to increasing the accuracy of such probes in determining the height of a feature.
BACKGROUND OF THE INVENTION
In semiconductor fabrication and related technologies, it has become necessary to routinely determine critical dimensions (CD) of physical features, usually in the horizontal dimension, formed in substrates. An example, shown in the illustrative cross sectional view of
FIG. 1
, includes a trench
10
formed in a substrate
12
, such as a silicon wafer. The illustration greatly exaggerates the depth of the trench
10
relative to the thickness of a silicon wafer
12
, but the illustrated aspect ratio of the trench
10
is realistic. In advanced silicon technology, the width of the trench may be 0.18 &mgr;m; and its depth, 0.7 &mgr;m. The critical dimension of the trench
10
may be the width of the top of the trench opening or may be the width of the bottom of the trench
10
. In other situations, the depth of the trench
10
is an important parameter. For the dimensions described above, the trench
10
has a high aspect ratio of greater than 4. Although typically sidewalls
14
of the trench
10
have ideal vertical profile angles of 90°, in fact the profile angle may be substantially less. Much effort has been expended in keeping the profile angle at greater than 85° or even 88° to 90°, but it requires constant monitoring of the system performance to guarantee that such a sharp trench is etched, and substantially lesser angles may be observed if sharp trenches are not required or the process has fallen out of specification. As a result, it has become necessary, either in the development laboratory or on the production line, to measure the profile of the trench
10
with horizontal resolutions of 0.18 &mgr;m or even substantially less. Depending upon the situation, the entire profile needs to be determined, or the top or bottom trench width needs to be measured. In other situations, not directly described here, the trench depth may be the critical dimension. More circular apertures, such as needed for inter-level vias, also need similar measurements. Similar requirements extend to measuring the profiles of vertically convex features such as interconnects.
To satisfy these requirements, profilometers based upon atomic force microscopy (AFM) and similar technology have been developed which rely upon the vertical position of a probe tip
20
, illustrated in
FIG. 1
, to measure critical dimension down to the order of tens to hundreds of nanometers. In the past, the probe tip
20
has assumed the form of a conical tip having atomically sized tip dimensions. Such a conical tip has difficulty reaching the bottom of a sharply sloping trench. More recently developed probe tips have a cylindrically or approximately square shaped cross sections of dimensions of 0.2 &mgr;m or less. Such a small probe tip is relatively short, of the order of micrometers, and is supported on its proximal end by a more massive tip support.
In the pixel mode of operation, the probe tip
20
is discontinuously scanned horizontally along a line. At multiple positions, which are typically periodically spaced but non-periodic spacing is possible, the horizontal scan of the probe tip
20
is stopped, and it is gently lowered until it is stopped by the surface of the substrate
12
. Circuitry to be briefly described later measures the height at which the probe tip stops. A series of such measurements around the feature being probed, for example, on either side of and within the trench
10
, provides a profile or topography of the sample.
In the related jumping mode of operation, the probe tip is continuously scanned while it is being lowered to the surface. Once the surface has been encountered, the vertical position of the probe tip is measured, and the tip is then raised during a continuation of the horizontal scan.
An example of such a critical dimension measurement tool is the Model JGCDM-12S available from Surface/Interface, Inc. of Sunnyvale, Calif. It employs technology similar to the rocking balanced beam probe disclosed by Griffith et al. in U.S. Pat. No. 5,307,693 and by Bryson et al. in U.S. Pat. No. 5,756,887. It is particularly useful in the above described pixel mode of operation. The tool is schematically illustrated in the side view of
FIG. 2. A
wafer
30
or other sample to be profiled is supported on a support surface
32
supported successively on a tilt stage
34
, an x-slide
36
, and a y-slide
38
, all of which are movable about their respective axes so as to provide two-dimensional and tilt control of the wafer
30
. Although these mechanical stages provide a relatively great range of motion, their resolutions are relatively course compared to the resolution sought in the probing. The bottom-slide
38
rests on a heavy granite slab
40
providing vibrational stability. A gantry
42
is supported on the granite slab
40
. A probe head
44
depends from the gantry
42
through an intermediate piezoelectric actuator providing about 10 &mgr;m of motion in (x, y, z). The piezoelectric actuator typically is a thin walled piezoelectric cylinder having separate x-, y-, and z-electrodes attached to the wall of the cylinder to thereby effect separately controlled movement along the three axes. A probe tip
46
projects downwardly from the probe head
44
to selectively engage the top surface of the wafer
30
and to determine its vertical and horizontal dimensions.
Principal parts of the probe head
44
of
FIG. 2
are illustrated in orthogonally arranged side views in
FIGS. 3 and 4
. A dielectric support
50
fixed to the bottom of the piezoelectric actuator
45
includes on its top side, with respect to the view of
FIG. 2
, a magnet
52
. On the bottom of the dielectric support
50
are deposited two isolated capacitor plates
54
,
56
and two interconnected contact pads
58
, which may be a single long film running between the capacitor pads
54
,
56
.
A beam
60
is medially fixed on its two lateral sides and electrically connected to two metallic and ferromagnetic ball bearings
62
,
64
. The beam
60
is preferably composed of heavily doped silicon so as to be electrically conductive, and a thin silver layer is deposited on it to make good electrical contacts to the ball bearings. However, the structure may be more complex as long as the upper surface of the beam
60
is electrically conductive in the areas of the ball bearings
62
,
64
and of the capacitor plates
54
,
56
. The ball bearings
62
,
64
are placed on the contact pads
58
and generally between the capacitor plates
54
,
56
, and the magnet
52
holds the ferromagnetic bearings
62
,
64
there. The attached beam
60
is held in a position generally parallel to the dielectric support
50
with a balanced vertical gap
66
of about 25 &mgr;m between the capacitor plates
54
,
56
and the beam
60
that allows a rocking motion of the 25 &mgr;m. Two capacitors are formed between the respective capacitor pads
54
,
56
and the conductive beam
60
. The capacitor pads
54
,
56
and the contact pads
58
, electrically connected to the conductive beam
60
, are connected to three terminals of external measurement and control circuitry to be described later. The beam
60
holds on its distal end a glass tab
70
to which is fixed a stylus
72
having the probe tip
20
projecting downwardly to selectively engage the top of the wafer
12
being probed. An unillustrated dummy stylus or substitute weight on the other end of the beam
60
provides rough mechanical balancing of the beam in the neutral position.
Three unillustrated electrical lines connect the two capacitor plates
54
,
56
and the contact pads
58
to a servo system that both measures the two capacitances and applies differential voltage to the two capacitor pads
54
,
56
to keep them in the balanced position. When the piezoelectric actuator
45
lowers the stylus
72
to the point that it encounters the feature

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