Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Utility Patent
1998-07-29
2001-01-02
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S216000, C250S306000, C250S307000, C073S105000
Utility Patent
active
06169281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for examining surface anomalies such as grooves and ridges in a sample surface, and, more particularly, to establishing the profiles of such anomalies using a scanning microscope dithered in mutually perpendicular lateral directions.
2. Description of the Related Art
The capability of measuring the profiles of circuit lines forming ridges on the surfaces of printed circuit boards and circuit chips, and the profiles of various types of very small trenches extending along substrate surfaces is becoming increasingly important in the field of micro-metrology. The alternative use of optical instrumentation has physical limitations for measuring objects smaller than one micron. Furthermore, the second alternative use of a scanning electron microscope has disadvantages caused by the difficulty of preparing the sample and performing measurements. The sample has to be sectioned before the measurements can be made, with measurements of a particular line or trench being made at only one location. Measurements must be taken in a vacuum environment, and the spatial accuracy of measurements is limited by effects resulting from interactions between the electron beam and the sample being measured.
FIG. 1
is a transverse cross sectional elevation of a trench in a sample surface
1
, together with a very sharp probe tip
2
, used in conventional scanning probe microscopy to determine surface features of the sample surface
1
. This probe tip
2
is vibrated in a direction, generally called the “Z-direction,” perpendicular to the overall surface
1
of the sample being measured, while relative motion between the probe
2
and the sample surface
1
is established along the surface of the sample in a scanning direction, such as the X-direction shown. At the end of a predetermined scanning motion, relative motion between the probe and the sample is established along the surface of the sample in a direction perpendicular to the scanning direction. This motion is used to begin a new scan line, lying parallel to the preceding scan line, so that a predetermined portion of the sample surface is traversed by a raster pattern. In scanning force microscopy, the probe tip
2
is fastened to the distal end of a cantilever, the proximal end of which is vibrated in the Z-direction at a constant amplitude and frequency. Under these conditions, the amplitude of the resulting vibration of the tip
2
depends on the level of engagement between the probe tip
2
and the sample surface
1
. Thus, a servo loop is established to move the proximal end of the cantilever in the Z-direction to maintain a constant amplitude of vibration of the probe tip
2
. Since the resulting movement of the proximal end of the cantilever follows variations of the sample occurring in the Z-direction as the probe is scanned in the X-direction, the driving signal generated in the servo loop to cause such movement is stored as a signal representing Z-direction variations in the sample surface
2
.
While these conventional methods are useful for examining a number of types of sample surfaces with relatively gradual upward and downward slopes, serious limitations are presented when such methods are used to examine surfaces having ridges and troughs, such as the trough
6
of FIG.
1
. The angle of inclination at which movement of the probe tip
2
can move upward or downward to follow the shape of the sample surface
1
is limited both by the fact that probe vibration is only in the Z-direction and by the physical shape of the probe. In the example of
FIG. 1
, the probe tip
2
, traveling in the X-direction, first contacts the upper edge
8
of an undercut wall surface
10
. As changes in the pattern of vibration indicate increased contact between the probe tip
2
and the sample surface
1
, the tip
2
is moved upward, remaining in contact with the edge
8
. Thus, the actual shape of the undercut wall surface
10
is not reflected in the movement of the probe tip
2
, which is used as a measurement of the surface
10
.
Furthermore, this conventional method introduces the possibility of a “crash” occurring between the probe tip
2
and an upward-extending wall surface, in the event that the probe
2
cannot be raised fast enough to clear the wall surface with continuing movement in the scan direction. Such an event can be expected to damage both the probe
2
and the sample surface
1
.
Thus, when the conventional method of
FIG. 1
is considered, what is needed is a method allowing the probe tip to follow the sample surface in spite of variations in the angle of inclination of wall surfaces and to prevent crashes between the probe tip and upward-extending surfaces.
A U.S. application, Ser. No. 08/861,118, filed May 21, 1997, now U.S. Pat. No. 5,801,381, having a common assignee with the present invention, the disclosure of which is hereby incorporated by reference, describes a method for preventing such a crash. In this method, the feedback signal developed to indicate the level of movement of the probe tip in the Z-direction needed to satisfy the predetermined condition of engagement between the probe tip and sample surface is compared with a stored threshold value corresponding to a maximum distance through which the probe tip can be driven in the Z-direction during an incremental portion of the scanning movement. The probe tip is then moved in the Z-direction according to the feedback signal, but scanning movement occurs only when the feedback signal is less than the stored value.
In view of the limitations of the conventional method of
FIG. 1
, a number of methods described in the patent literature have been developed for using scanning probe microscopy to measure the profiles of wall surfaces.
For example, U.S. Pat. No. 5,186,041 to Nyyssonen describes a metrology system for measuring the depth and width of a trench in a sample to be tested with a probe moved relative to the sample. The system detects the proximity of the probe to a surface forming the bottom of the trench and to the sidewalls of the trench. The system adjusts the relative position of the probe and the sample vertically and transversely as a function of the output signals.
FIG. 2
is a lateral elevation of the probe
12
, described in U.S. Pat. No. 5,186,041, which has three protuberances to detect the depth and width of the trench. A first protuberance
14
extends downward to sense the bottom of a trench. Lateral protuberances
16
extend in opposite directions, across the width of the trench, from the probe, to detect the side walls of the trench. The apparatus associated with this probe
12
has means for vibrating the probe in either the Z-direction or in the X-direction, together with interferometric apparatus for measuring vibrations in both the Z- and X-directions.
FIG. 3
is a transverse cross-sectional view of a sample surface
18
including a trench
20
, with dashed lines
22
indicating movements of the probe of
FIG. 2
, used tomeasure the trench. After the surface heights at each side of the trench are measured at points
24
, the probe tip
12
is driven downward while being vibrated in the Z-direction to measure the depth of the trench at a central point
26
. Next, the probe
12
moved upward through incremental distances, and is alternately driven against each of the opposite sidewalls
28
, with the probe being vibrated in the X-direction, as measurements are made at points
30
.
U.S. Pat. No. 5,321,977 to Clabes et al describes the use of an integrated tip strain sensor in combination with a single-axis atomic force microscope (AFM) for determining the profile of a surface in three dimensions. A cantilever beam carries an integrated tip stem on which is deposited a piezoelectric film strain sensor. A piezoelectric jacket with four superimposed elements is deposited on the tip stem. The piezoelectric sensors function in a plane perpendicular to that of a probe in the atomic force microscope; that is, any tip contact with a sidewall surface causes tip deflec
Chen Dong
Flecha Edwin
Hammond James Michael
Martin Yves Corfield
Roessler Kenneth Gilbert
Davidge Ronald V.
International Business Machines - Corporation
Lee John R.
Tomlin Richard A.
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