Method and system for non-contact measurement of microwave capacitance of miniature structures of integrated circuits
A method and system for non-contact measurements of microwave capacitance of miniature structures patterned on wafers used for production of modern integrated circuits. A near-field balanced two-conductor probe is brought into close proximity to a test key built on a wafer of interest and replicating the miniature structure of interest. The resonant frequency of the probe for the test key is measured. The probe is then positioned at the same distance from an “open” calibration key and “short” calibration key, and the resonance frequencies of the probe for the calibration keys are measured. A shear force distance control mechanism maintains the distance between the tip of the probe and the measured test key and calibration keys. The microwave capacitance of the test key is then calculated in accordance with a predefined formula. The obtained microwave capacitance of the test key is indicative of the capacitance of the miniature structure of interest and may be further used for determining possible defects of the integrated circuit of interest.
1. A method for non-contact measurement of microwave capacitance of miniature structures of interest of integrated circuits under study, comprising the steps of:
building at least one test key on a wafer of interest, said at least one test key including first pads and a test key portion coupled to said first pads, said test key portion replicating the miniature structure of interest of the integrated circuit, the dimensions of said test key portion falling in a predetermined range of dimensions corresponding to the dimensions of said miniature structure of interest;
positioning a near-field microwave probe at a predetermined distance from said first pads of said at least one test key;
measuring a resonant frequency F tot of said near-field microwave probe for said at least one test key;
building at least one “short” calibration key on the wafer of interest, said “short” calibration key including second pads coupled each to the other by a conducting portion;
positioning said near-field microwave probe said predetermined distance from said second pads of said at least one “short” calibration key;
measuring a resonant frequency F sh of said near-field microwave probe for said at least one “short” calibration key;
building at least one “open” calibration key on the wafer of interest, said “open” calibration key including third pads isolated one from another;
positioning said near-field microwave probe said predetermined distance from said third pads of said at least one “open” calibration key;
measuring a resonant frequency F op of said near-field microwave probe for said at least one “open” calibration key; and
calculating the microwave capacitance C LL of said at least one test key as
C
LL
=
(
1
-
2
F
sh
L
ɛ
0
μ
0
ɛ
eff
)
2
2
Z
0
(
F
op
-
F
tot
)
(
F
op
-
F
sh
)
(
F
sh
-
F
tot
)
wherein F op is the resonant frequency of said near-field microwave probe for the “open” calibration key, F sh is the resonant frequency of said near field microwave probe for the “short” calibration key, F tot is the resonance frequency of said near-field microwave probe for the test key, Z 0 is the characteristic impedance of said near-field microwave probe, L is the resonator length, ε 0 is the absolute electric permittivity of a vacuum, μ 0 is the absolute permeability of a vacuum, and ε eff is the effective dielectric constant of the transmission line that forms said near-field microwave probe.
2. The method of claim 1 , wherein said test key portion includes at least a pair of test key wires and wherein said capacitance C LL is an interlines or line-to-line capacitance.
3. The method of claim 2 , further comprising the step of:
building a plurality of said test keys in proximity each to the other, said test keys having distinctive lengths of said test key wires ranging from 5 μm to 50 μm.
4. The method of claim 3 , further comprising the steps of:
measuring said capacitance C LL for said plurality of test keys having said test key portions of different dimensions, and
calculating a specific capacitance C LL per unit length of said test key portions.
5. The method of claim 2 , wherein the width of said test key wires is approximately 0.15 μm, and spacing between said test key wires is in the range of 0.1–1 μm.
6. The method of claim 1 , wherein said test key portion includes at least a pair of wires extending to form a comb-like inter-digital capacitor.
7. The method of claim 1 , wherein said near-field microwave probe includes a balanced two-conductor transmission line resonator.
8. The method of claim 1 , wherein said near-field microwave probe includes at least a pair of line conductors extending in spaced relationship therebetween and separated by a dielectric medium.
9. The method of claim 8 , wherein said first, second and third pads have substantially the same dimensions, said dimensions being determined by dimensions of said at least pair of line conductors.
10. The method of claim 9 , wherein said near-field microwave probe includes a tip, wherein an area of said first, second and third pads is at least the size of a cross-section of said tip of said near-field microwave probe.
11. The method of claim 9 , wherein the length of each said first, second and third pads is in the range of 5–20 μm, the width of each said first, second and third pads is in the range of 2–6 μm, and the separation between the pads in said first, second and third pads is in the range of 1–10 μm.
12. The method of claim 1 , further comprising the steps of:
positioning said at least one test key at the scribe line on said wafer of interest.
13. The method of claim 1 , further comprising the steps of:
positioning said at least one “short” and said at least one “open” calibration keys in proximity to said at least one test key.
14. The method of claim 1 , further comprising the steps of:
maintaining said predetermined distance between said near-field microwave probe and said at least one test key equal to said predetermined distance between said near-field microwave probe and said at least one “short” and said at least one “open” calibration keys by a closed-loop distance control mechanism.
15. The method of claim 14 , wherein said closed-loop distance control mechanism includes a shear force-based distance control mechanism.
16. The method of claim 1 , wherein said predetermined distance is maintained below 50–100 nm.
17. The method of claim 1 , further comprising the steps of:
comparing said calculated microwave capacitance C s with a predetermined capacitance value, and judging whether said miniature structure is defective based on a deviation of said C s from said predetermined capacitance value.
18. A system for non-contact measurement of microwave capacitance of miniature structures of integrated circuits, comprising:
a miniature structure of interest located on a wafer under test;
a near-field microwave probe having a tip thereof;
at least one test key built on said wafer under test and including first metallic pads and a test key portion replicating said miniature structure of interest;
at least one “open” calibration key built on said wafer under test and including second metallic pads isolated one from another;
at least one “short” calibration key built on said wafer under test and including third metallic pads shorted each to the other by a conductive element;
a distance control unit operatively coupled to said near-field microwave probe to control a distance from said probe tip to said first, second and third metallic pads;
acquisition means for acquiring resonant frequency F tot of said near-field microwave probe for said at least one test key, resonant frequency F sh of said near-field microwave probe for said “short” calibration key, and resonant frequency F op of said field microwave probe for said “open” calibration key; and
processing means for calculating the microwave capacitance C LL of said test key as
C
LL
=
(
1
-
2
F
sh
L
ɛ
0
μ
0
ɛ
eff
)
2
2
Z
0
(
F
op
-
F
tot
)
(
F
op
-
F
sh
)
(
F
sh
-
F
tot
)
wherein F op is the resonant frequency of said near-field microwave probe for the “open” calibration key, F sh is the resonant frequency of said near field microwave probe for the “short” calibration key, F tot is the resonance frequency of said near-field microwave probe for the test key, Z 0 is the characteristic impedance of said near-field microwave probe, L is the resonator length, ε 0 is the absolute electric permittivity of a vacuum, μ 0 is the absolute permeability of a vacuum, and ε eff is the dielectric constant of the transmission line that forms said near-field microwave probe.
19. The system of claim 18 , wherein said near-field microwave probe includes at least a pair of conductors extending in spaced relationship therebetween and separated by a dielectric media, and wherein said first, second and third metallic pads have dimensions determined by dimensions of said at least pair of conductors.
20. The system of claim 18 , further comprising a plurality of said test keys built in proximity each to the other on a test zone of said wafer under study, said test keys having distinct test key portions having geometric properties covering the range of the geometric features of a plurality of miniature structures of interest.