Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-11-09
2002-12-03
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S600000
Reexamination Certificate
active
06487910
ABSTRACT:
SUMMARY OF THE INVENTION
This invention relates to non-destructive testing methods and apparati for determining the “cleanliness,” that is, degree of material internal purity, of metallic sputter target materials and, more particularly, non-destructive methods and apparati for determining cleanliness based on the sound propagation properties of the materials.
BACKGROUND OF THE INVENTION
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates such as semiconductor wafers. Basically, a cathode assembly including a sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits on the receiving surface of the substrate to form the thin layer or film.
One factor affecting the quality of the layer or film produced by a sputtering process is the “cleanliness” of the material from which the sputter target is made. The term “cleanliness” is widely used in the semiconductor industry, among others, to characterize high purity and ultra high purity materials. In common practice, “cleanliness” refers to the degree of material internal purity. Such impurities may be present, for example, as inclusions of impurity-rich phases surrounded by the sputter target material. Cleanliness is usually measured in units of particles per million (“ppm”) or particles per billion (“ppb”) which define a ratio between the number of contaminant atoms and the total number of atoms sampled.
Since the cleanliness of the material from which a sputter target is made affects the quality of layers of films produced using that target, it is obviously desirable to use relatively clean materials in fabricating sputter targets. This implies a need in the art for non-destructive techniques for selecting sputter target blanks of suitable cleanliness to produce high quality sputter targets. Known destructive test methods, such as glow discharge mass spectroscopy and LECO techniques, are not suitable for this purpose.
Another factor affecting the quality of the layer or film produced by a sputtering process is the presence of “flaws” in the sputter target material. As used herein, the term “flaws” refers to microscopic volumetric defects in the sputter target material, such as inclusions, pores, cavities and micro-laminations. Since flaws in a sputter target affect the quality of layers or films produced using that target, there exists a corresponding need in the art for non-destructive techniques for characterizing flaws present in sputter target materials.
FIG. 1
illustrates a prior art non-destructive ultrasonic “flaw” detection method for characterizing aluminum and aluminum alloy sputter target materials. The technique illustrated in
FIG. 1
is similar to that suggested in Aluminium Pechiney PCT Application No. PCT/FR96/01959 for use in classifying aluminum or aluminum alloy blanks suitable for fabricating sputter targets based on the size and number of internal “decohesions” detected per unit volume of the blanks.
The prior art technique of
FIG. 1
employed a pulse-echo method performed on a test sample
10
having a planar upper surface
12
and a parallel planar lower surface
14
. In accordance with this technique, focused ultrasonic transducer
16
irradiated a sequence of positions on the upper surface
12
of the test sample
10
with a single, short-duration, high-frequency ultrasound pulse
18
having a frequency of at least 5 MHz, and preferably 10-50 MHz. The ultrasonic transducer
16
then switched to a sensing mode and detected a series of echoes
20
induced by the ultrasound pulse
18
.
One factor contributing to these echoes
20
was scattering of sonic energy from the ultrasound pulse
18
by flaws
22
in the test sample
10
. By comparing the amplitudes of echoes induced in the test sample
10
with the amplitudes of echoes induced in reference samples (not shown) having compositions similar to that of the test sample
10
and blind, flat-bottomed holes of fixed depth and diameter, it was possible to detect and count flaws
22
in the test sample
10
.
The number of flaws detected by the technique of
FIG. 1
had to be normalized in order to facilitate comparison between test samples of different size and geometry. Conventionally, the number of flaws was normalized by volume—that is, the sputter target materials were characterized in units of “flaws per cubic centimeter.” The volume associated with the echoes
20
from each irradiation of the test sample
10
was determined, in part, by estimating an effective cross-section of the pulse
18
in the test sample
10
.
One drawback to the technique of
FIG. 1
is that a number of factors detract from the ability of the transducer
16
to detect sonic energy scattered by the flaws
22
. This reduces the sensitivity of the technique.
One such factor is relative weakness of the scattered energy. A portion of the scattered energy is attenuated by the material making up the test sample
10
. Furthermore, since the flaw sizes of interest, which range from approximately 0.04 mm to 0.1 mm, are significantly less than the wavelength of ultrasound in metals (for example, the wavelength of sound in aluminum for the frequency range of 10 MHz to 50 MHz is 0.6 mm to 0.12 mm, respectively), the pulse
18
has a tendency to refract around the flaws
22
, which reduces the scattering intensity.
Another factor detracting from the ability of the transducer
16
to detect the sonic energy scattered by the flaws
22
is the noise generated by scattering of the pulse
18
at the boundaries between grains having different textures. In fact, the texture-related noise can be so great for high-purity aluminum having grain sizes on the order of several millimeters that small flaws within a size range of approximately 0.05 mm and less cannot be detected. Larger grain sizes reduce the signal-to-noise ratio for the sonic energy scattered by the flaws when compared to the noise induced by the grain boundaries.
Other factors affecting the sensitivity and resolution of the technique of
FIG. 1
includes the pulse frequency, duration and waveform; the degree of beam focus and the focal spot size; the coupling conditions (that is, the efficiency with which the sonic energy travels from the transducer
16
to the test sample
10
); and the data acquisition system parameters.
Another drawback to the technique of
FIG. 1
is that the calculation of the “flaws per cubic centimeter” in the test sample
10
presupposes that only flaws
22
within a determinable cross-sectional area scatter sonic energy back toward the transducer
16
. In fact, the pulse
18
, due to its wave nature, does not have localized, well-determined boundaries.
The distribution of the energy of the pulse
18
within the test sample
10
, under simplifying assumptions, permits one to define a corridor
30
having a determinable cross-section beneath the transducer
16
in which most of the energy should be concentrated. Nevertheless, some of the energy of the pulse
18
will propagate outside this corridor
30
. As a result, the transducer may detect sonic energy scattered by relatively large flaws
22
located outside the estimated corridor
30
, thereby overestimating the density of flaws
22
in the test sample
10
and underestimating their sizes. Therefore, material cleanliness characteristics become to some degree uncertain.
Thus, there remains a need in the art for non-destructive techniques for characterizing sputter target materials having greater sensitivity than
Biebel & French
Moller Richard A.
Tosoh SMD, Inc.
LandOfFree
Method and apparatus for quantitative sputter target... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for quantitative sputter target..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for quantitative sputter target... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2990787