Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
2000-09-08
2003-03-25
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
C324S639000, C324S642000, C324S631000
Reexamination Certificate
active
06538454
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to electrical measuring equipment and methods. In particular, the invention relates to microwave equipment and methods for mapping resistivity, thickness, and other electrical characteristics over a surface with resolution of a few micrometers, that is, a microwave microscope.
BACKGROUND ART
In U.S. Patent 5,781,018, two of us, Davidov and Golosovky, describe a microwave microscope including a single resonant slit. We describe an improvement of this microwave microscope in U.S. Pat. No. 6,100,703 including a crossed pair of resonant slits. Both these patents are incorporated by reference in their entireties.
The prior-art microscope of U.S. Pat. No. 5,781,018, as illustrated schematically in
FIG. 1
, includes a microwave probe
12
formed of a rectangular waveguide
14
, the probe end of which is covered with a thin conductive foil
16
. A resonant slit
18
, to be described in more detail later, is formed in the conductive foil
16
. The probe end is positioned over a front surface
20
of a sample
22
to be scanned. For example, the sample
22
may be a silicon wafer covered by metallic and/or dielectric layers and having various very small features formed in its surface
20
which need to be electrically characterized. The sample
22
is mounted on an X-Y stage
26
driven by a drive
28
under the control of a computer
30
so as to allow the probe
12
to be scanned over the sample surface
20
. A source
32
of microwave radiation, provides the probing microwaves to a microwave bridge formed of a hybrid tee, an adjustable attenuator
36
, a sliding short
38
, and an E-H tuner
40
that matches the impedance of the probe antenna (slit)
18
to that of the waveguide
14
. A microwave detector
42
receives radiation from the bridge to thereby measure its imbalance, and the intensity is transmitted to the computer
30
. The amount of imbalance is determined, in part, by the electrical characteristics of the material in the sample
22
immediately below the slit
18
, and thus can be used to measure those electrical characteristics on a scale equal to the dimensions of the slit. In the patent, the microwave radiation is in the millimeter band, about 80 GHz.
In a preferred embodiment illustrated orthographically in
FIG. 2
, the probe
12
is formed with a one-dimensionally curved foil end
16
. The long dimension of the slit
18
extends along the curve, and the short dimension of the slit
18
is transverse to the curve. As is described in detail in the two patents, if the long dimension of the slit
18
is made nearly resonant with the probing radiation, that is, half of a free-space wavelength (a few millimeters at 80 GHz), the slit's short dimension can be made nearly arbitrarily small, but the probe end remains nonetheless nearly transparent to the microwave radiation. If the slit's short dimension is formed to the order of micrometers or somewhat less, the short dimension defines the sampling resolution of the probe along the transverse direction of the slit.
The mechanical stability of the convexly curved probe end is improved, as illustrated in the side cross-sectional view of
FIG. 3
, by placing a low-loss dielectric body
44
at the end of the probe. Its curved front end supports the thin foil
16
. Its sides fit closely to the rectangularly shaped waveguide
14
. Its pyramidally shaped back end minimizes microwave reflection. To further minimize microwave reflection, the dielectric body should have a low dielectric constant, for example, &egr;=2.2. U.S. Pat. No. 5,781,018 also teaches dielectrically loading the entire microwave waveguide.
The U.S. Pat. No. 6,100,703 includes two resonant slits arranged perpendicularly. Polarization-sensitive detection equipment then allows separated detection of the incident polarization and the perpendicular polarization. The rotated polarization (90° rotated) is particularly useful for mapping Hall mobilities, anisotropies, and local non-uniformities.
An important task for the near-field microwave microscope is the contactless characterization of conductive layers, particularly thickness mapping of thin metal layers overlying a dielectric underlayer. For conductive films that are thinner than the skin-depth of their constituent metal, the characterization may be conveniently performed through the local measurement of the sheet resistance R
sh
=p/t, where p is the bulk resistivity of the metal and t is the thickness of the metal layer. The metal layers most used in semiconductor fabrication are composed of Cu, Ag, Al, and W and have thicknesses in the range of 0.1 to 3 &mgr;m and sheet resistances in the range of 0.1 to 10 &OHgr;. The skin-depths for these materials at microwave frequencies between 1 and 100 GHz are in the range of 0.2 to 1 &mgr;m. Thus, a microwave microscope of proper design can in many cases characterize the sheet resistance and thus thickness of these metal layers. In the fabrication of semiconductor integrated circuits, it is often important to determine the uniformity of metal deposition to assure a sufficiently thick metal layer on all portions of the wafer.
However, the use of a microwave microscope for conductive layer of appreciable thickness, even of a significant fraction of a skin depth, requires the use of low impedance probe. The sensitivity of a simple slit probe and of most popular microwave probes is such that they allow the characterization of conductive films with sheet resistances of 100 &OHgr; or greater. This sensitivity is not enough to effectively probe metal layers of sheet resistance of less than 10 &OHgr;. Hao et al. have disclosed a low-impedance scanning dielectric resonator in “Spatially resolved measurements of HTS microwave surface impedance,” IEEE
Transaction in Applied Superconductivity
, vol. 9, no. 2, Jun. 1999, pp. 1944-1947. However, their resolution of a few millimeters is not fine enough for characterizing the small features of modern semiconductor integrated circuits.
Accordingly, it is desired to provide a probe capable of electrically characterizing conductive layers on a semiconductor wafer. In particular, it is desired to provide such a probe for layers having sheet resistance of less than 10 ohms.
It is further desired to provide a microwave probe antenna that has a narrow slit and a low electrical impedance.
SUMMARY OF THE INVENTION
A microwave microscope including a narrow resonant slit in a conductive end of the probe tip and a dielectric resonator in the waveguide behind the resonant slit to impedance match the waveguide to the high impedance slit. The dielectric resonator is formed by a resonator member having a high dielectric constant, placed next to the resonant slit, and having a resonant length, of the order of the microwave radiation in the material. A long dielectric member is placed in back of the resonator member and separated from the resonator member by a small gap having a width chosen to form an impedance transformer matching the waveguide impedance to the impedance of the combination of the slit and resonator member. The gap width preferably is in the range of 0.1% to 100% of the free-space wavelength of the microwave radiation. The gap may be operationally set by varying its length to minimize microwave reflection from assembly of the slit and the resonator member.
The front end of the resonator member, that is, the end facing free space may be flat or preferably convex. The conductive end of the waveguide and the slit may be formed by coating this front surface of the resonator member with a metal layer and forming the slit in the coated metal.
REFERENCES:
patent: 5781018 (1998-07-01), Davidov et al.
patent: 5821410 (1998-10-01), Xiang et al.
patent: 5900618 (1999-05-01), Anlage et al.
patent: 6020800 (2000-02-01), Arakawa et al.
patent: 6100703 (2000-08-01), Davidov et al.
patent: 6209482 (2001-04-01), Doehler
patent: 6376836 (2002-04-01), Anlage et al.
patent: 2002/0041221 (2002-04-01), Abdulnour
Poole, “A comprehensive treatise on experimental techiniques”,E
Davidov Dan
Frenkel Avraham
Golosovsky Michael
Guenzer Charles S.
Sundaram T. R.
Yissum Research Development Company of the Hebrew University Jer
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