Method of mapping a surface using a probe for stylus...

Measuring and testing – Surface and cutting edge testing – Roughness

Reexamination Certificate

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C073S001890, C216S011000, C250S306000, C250S307000

Reexamination Certificate

active

06250143

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to a semiconductor feature measurement apparatus and a method of manufacturing that apparatus and, more specifically, to a method of manufacturing probes for a stylus nanoprofilometer having a non-circular probe tip morphology and to a method of measurement of semiconductor wafer features using the same.
BACKGROUND OF THE INVENTION
As line widths and features within the semiconductor industry continue to decrease in size there is an ever-increasing need to discover new ways and tools to accurately define the size and shape of the features in a microcircuit. Critical dimensions and accurate formation of various devices within an integrated circuit are paramount in producing high quality semiconductor devices, and the scanning electron microscope (SEM) has long been an industry standard for examining such features. The SEM uses a very fine probing beam of electrons that sweeps over the surface of the specimen causing the surface to emit a variety of radiations. Measuring the radiation creates a signal that is proportional to the amount of radiation leaving an individual point of the specimen at any instant. This signal can be used to modulate the brightness of a display cathode-ray tube (CRT) as an illumination beam rests on a corresponding pixel of the CRT image. In practice, the pixels follow one another with great rapidity so that the image of each pixel becomes an image of a line, and the line, in turn, becomes a series of lines that move down the screen so rapidly that the human eye sees a complete image. The CRT image can also be recorded in its entirety by allowing the pixel-by-pixel information to build up in sequence on a photographic film.
As semiconductor features continue to decrease in size, now reaching less than 200 nm and projected to reach to about 120 nm, it is becoming increasingly important to have the ability to measure the actual features formed on a semiconductor wafer. The SEM has historically been an excellent analytical tool for determining the nature, width, and length of features on the upper surface of a semiconductor die. In the early 1990's the SEM was adequate for detailed feature analysis because feature size was on the order of 500 nm and larger. As feature sizes continue to decrease, the exact nature of the sidewall becomes increasingly important. However, a SEM beam that is vertical, i.e., with respect to the die surface, has significant difficulty in determining the depth of some features in today's sub-250 nm feature sizes.
To illustrate the problem of a vertical SEM on a very small surface, refer initially to FIG.
1
. Illustrated is a sectional view of one embodiment of a simple semiconductor wafer feature
100
being subjected to a vertical electron beam, collectively
105
, of a SEM (not shown). The illustrated semiconductor wafer feature
100
comprises, a first upper surface
111
, a lower surface
120
, first and second sides
131
,
132
, and a second upper surface
112
. The first and second sides
131
,
132
are shown as they are typically found. That is, the sides
131
,
132
are not exactly vertical, but rather slightly outward sloping (note angles
131
a
and
132
a
), with respect to the lower surface
120
. In prior years, the wall slope, i.e., typically angles
131
a,
132
a
of perhaps 0.5 to 3 degrees off the vertical, of channel features was known, but was not significant when considered against a total width
101
and depth
102
of the feature
100
.
While the planar location (x and y coordinates) of any point on a surface of a feature can readily be ascertained from the stepper mechanism that operates the electron beam
105
, the vertical location (z coordinate) of the point may be problematic. As the vertical electron beam
105
of a SEM passes from left to right, i.e. passes through positions
105
a
through
105
m
sequentially, the first upper surface
111
is readily defined by the beam
105
at positions
105
a
through
105
c.
However when the electron beam
105
passes over the first side
131
, that is, from
105
d
through
105
f,
there can be an uncertainty as to the depth of the surface
131
being impacted by the electron beam
105
. An edge effect causes secondary electrons
106
to be generated when the electron beam
105
d,
105
e
strikes a corner
133
of sloping first side
131
and causes what is called a “blooming effect” in the image. As with the first upper surface
111
, the lower surface
120
is readily discerned by the electron beam
105
g
-
105
i,
but the blooming effect re-occurs on the second side
132
at positions
105
j
through
105
l
. This disrupts how finely the sidewall
131
,
132
depth can be determined. With wall slopes as mentioned, the morphology of the wall where the electron beam
105
is striking becomes clouded as the secondary emission
106
of electrons from the target blooms. It therefore becomes uncertain as to the exact shape and dimensions of the side walls
131
,
132
.
Thus, a vertical SEM is limited in usefulness for analyzing an existing feature. To effectively use the SEM for feature depth measurements, the semiconductor die must be sectioned, allowing SEM to be performed on the section, rather than vertically from the upper surface. This allows what could be termed a horizontal SEM, i.e., a SEM oriented into the plane of FIG.
1
. However, sectioning results in destruction of the wafer, and is therefore undesirable.
Another negative factor with SEM examination is that it charges the surface being examined, that is, electrons are bombarded onto the surface of the sample, and secondary emissions from the target are then measured. Thus, the scanning electron microscope has about reached its limit in its ability to provide information on the semiconductor features being formed today. Therefore, one might reasonably prefer to have a non-intrusive examination method that does not interact with the sample or its surface.
In light of the aforementioned problems, one approach to a solution might be to use a physical measurement system, bypassing the intrusive nature of the SEM, as well as eliminating a need for sectioning the semiconductor die. One such existing tool is a stylus nanoprofilometer (SNP), also know as a critical dimension atomic force microscope (CDAFM). Referring now to
FIG. 2
, illustrated is a schematic representation of a conventional, single-direction balance beam force sensor
200
. The SNP (not shown) uses the balance beam force sensor
200
to monitor a force
215
between a probe tip or stylus
210
and a sample surface
220
. Additional information on balance beam force sensors may be obtained in “Dimensional Metrology with Scanning Probe Microscopes”, Journal of Vacuum Science and Technology Bulletin 13, pg 1100, pub. 1995, incorporated herein by reference. By monitoring a change in capacitance at locations
251
,
252
between a scan actuator
230
and a balance beam
240
, contact with the surface
220
can be detected. Referring now to the enlarged view, by moving the probe tip
210
from point to point on the sample surface
220
, one who is skilled in the art will readily understand that the topography of the surface
220
can be mapped.
Of course, different problems present themselves when using physical means, rather than a SEM, for device measurements. For critical dimension measurements, the shape of a mechanical probe tip, which is of course a finite size, must be extracted from the obtained data. Therefore, mechanical probe tips must be: (a) made so that they are easily characterized, and (b) have only one proximal point, that is, one point of interaction between the sample and the probe tip.
Referring now to
FIGS. 3A and 3B
, illustrated are elevational views of probative portions
311
,
321
of conventional cylindrical and conical probes
310
,
320
, respectively, for a stylus nanoprofilometer. In the early
1990
's, cylindrical and conical probe tips could be made by chemically etching a single strand of optic fiber to form a te

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