Cleanliness evaluation in sputter targets using phase

Details Cleanliness evaluation in sputter targets using phase Cleanliness evaluation in sputter targets using phase

2002-10-31

2004-05-25

Williams, Hezron (Department: 2856)

Measuring and testing

Vibration

By mechanical waves

C073S598000

Reexamination Certificate

active

06739196

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to non-destructive testing methods and apparatus for identifying types of intrinsic flaws in metallic sputter target materials and, more particularly, non-destructive methods and apparatus for identifying and counting of solid inclusions using radio frequency echo waveform phase change detection.
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 normal 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 traces of foreign elements in distributed or localized form in 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. However, not all the flaws are “alike” in their degrading effect on sputter performance. Some types of flaws, for example, micro-cavities or shrinkage porosity cause relatively “mild” degrading effect on sputter performance while the others, such as dielectric inclusions, cause a serious disturbance in the sputter process. Therefore, there exists a corresponding need in the art for a non-destructive technique which identifies and separately counts different kinds of flaws which may exist 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 Aluminum 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, a 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
.
A portion of the scattered energy is attenuated by the material making up the test sample
10
. Furthermore, since the single flaw sizes of interest, which range from approximately 0.04 mm to 0.8 mm, are of same range with 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 physical nature of the substance of the flaw or more accurately a degree of acoustic impedance mismatch at the flaw—matrix material boundary. The impedance mismatch directly affects the reflection and transmission characteristics of ultrasound at the phase boundaries. The reflection coefficient of ultrasound beam at matrix-to-flaw boundary can be expressed by the simplified expression: R=(I
2-−I
1
)/(I
2
+I
1
), where I
2
is an acoustic impedance of the flaw material, and I, is an acoustic impedance of the matrix material. The simple analysis of this formula allows us to derive several important conclusions. At first, if acoustic impedance of the flaw I
2
is less than the acoustic impedance of the matrix I
1
, then the R coefficient becomes negative. The negativity of the R can be translated as a change in the phase of the acoustic pulse waveform on 180°. For example, if the flaw is the gas-filled or vacuumed (shrinkage) void with the acoustic impedance equal to 0.93 g/cm
2
-sec(×10
6
) (air) or below (vacuum), then the phase of the ultrasound pulse waveform is changed on 180° at the boundary. At second, if the flaw is a gas filled or vacuumed void in the aluminum matrix with the acoustic impedance of 17.2 g/cm
2
-sec(×10
6
), then the reflection coefficient value is close to the unity or 100% and the amplitude of the reflected signal is the only function of the relationship between flaw size and the ultrasound beam focal spot size. At third, if the flaw comprises a solid particle, for example, an alumina inclusion with the acoustic impedance of 39.6 g/cm
2
-sec(×10
6
), which exceeds the acoustic impedance of the aluminum matrix more than two times (17.2 g/cm
2
sec(×10
6
)), the ultrasound waveform does not experience the phase inversion at the flaw boundary, and for alumina inclusion the reflection coefficient does not exceed 39.5% of the amplitude of the impinging pulse (if the wave interference effect is not considered). In this case, the amplitude of the

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