Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-09-02
2004-02-10
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S305000, C250S306000, C250S307000, C250S311000, C250S397000, C378S044000, C378S045000, 36, 36
Reexamination Certificate
active
06690010
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a method for determining the chemical composition of fine features, such as surface particles on semiconductor wafers. During the manufacturing test of semiconductor wafers, the location of small defects may be detected using a variety of methods. Frequently, the composition of the defect yields important information about the manufacturing process. Such information may assist, for example, in isolating problems in the manufacturing process or discovering contamination from errant debris in the manufacturing environment.
Others have proposed determining the composition of small defects using “energy-dispersive spectroscopy”, or EDS, as described in relation to FIG.
1
. In an EDS system
10
, an incident electron beam
20
is directed towards a known defect location
22
at a fixed energy, generally 5-20 keV. X-rays
24
are emitted from the defect particle and detected by an x-ray detector
30
. Chemical identification of a defect is made by resolving the energy of x-rays emitted from the sample as a result of the irradiation.
The incident beam
20
in EDS system
10
has a fixed energy and current, and all of the x-rays emitted by the sample are detected simultaneously by detector
30
and their energies are resolved. X-ray emissions occur only when the incident electron energy exceeds a minimum threshold energy level sufficient to knock electrons out of particular levels.
FIG. 2
shows a diagram plotting the intensity of the x-rays detected by detector
30
across x-ray energies for a sample including aluminum (Al), silicon (Si), copper (Cu) and tungsten (W), all commonly used in the manufacture of semiconductor devices. Around the x-ray energy level of Al a peak intensity
50
indicates an increased level of x-rays emitted from the sample location. Similarly a peak intensity
54
is indicated around the energy level of Cu.
A smaller intensity peak
52
is formed around the area of energy level of Si. The difference between the x-ray energy level of Al and Si is only about 250 eV. Because the resolving power of the detectors used in EDS is never better than 50 eV, and is frequently worse than 150 eV, a great deal of data must be collected in order to resolve two peaks so close together.
EDS systems also present another problem. For most atomic species, the 5-20 keV supplied by the incident beam
20
of an EDS system is much greater than that necessary to excite the electrons, so the depth of material probed is usually 0.5 to 5 microns. However, a small defect particle may be as little as 0.1 microns deep. For a small defect particle
22
, as shown in
FIG. 1
, the beam may therefore probe the substrate
12
as well as the defect
22
. As beam
20
penetrates the substrate
12
, the electrons of the beam are deflected throughout a volume
26
, that may be 1-10 microns wide, exciting other electrons in the volume. X-rays
28
are emitted from atoms throughout the probed volume. It is impossible to discern between the x-rays emitted from the particle or from the substrate, so it cannot be determined if any chemicals detected are from the errant particle, or are from the substrate. Some have solved this problem by testing at the particle location, and testing a second time near the particle area to resolve the difference, but this process requires additional processing time and may introduce other errors if the neighboring area tested has different structures from the defect area.
Another technique that has been used by others to determine the chemical composition of defects is Auger Electron Spectroscopy (AES), illustrated by system
70
, shown in FIG.
3
. In an AES process, a fixed electron beam
72
of 3-5 keV is projected towards particle
22
on substrate
12
. Auger electrons
74
are emitted from particle
22
and detected by an electron energy analyzer
76
. The energy of Auger electrons is well known, and is fixed for each atomic species.
Although the beam
72
may penetrate the particle
22
and substrate
12
, Auger electrons, unlike x-rays, are only released from the surface area struck by the beam. An AES system effectively probes only about 0.005-0.05 microns into the particle. Although this solves the problem in EDS of probing the substrate below the defect particle, it raises additional implementation problems. Since such a shallow amount of the surface is probed, it is very important that the surface is not contaminated with even a minute residue of other material. For example, condensed water in the air will affect the chemical determination. This problem has been reduced by cleaning the surface by ion-bombardment
82
, and holding the material in an ultra-high vacuum chamber
80
at 10
−9
Torr or lower before measurements are made.
The specialized equipment needed for AES systems—ultra high vacuum system and electron energy analyzer—is very expensive, and requires highly-trained operators. Auger systems, because of the need for ultra-high vacuum control, also cannot be easily retrofit to existing manufacturing equipment. AES is therefore not suitable for in-line analysis of semiconductor product wafers, and is instead commonly used for failure analysis of semiconductor devices.
A third method employs variations of electron appearance spectroscopy to the problem of identifying materials. For example, Park and Houston, in “APPEARANCE POTENTIAL SPECTROSCOPY ON AN AUSTERE BUDGET”, SURFACE SCIENCE, 26 (1971) (pages 664-666), Letters to the Editor, describe a simple appearance spectroscopy system in which the derivative of the photocurrent as a function of the sample potential exhibits sharp peaks at the potentials corresponding to the threshold energies. Chopra and Chourasia in “APPEARANCE POTENTIAL SPECTROSCOPY OF SOLID SURFACES” SCANNING MICROSCOPY, Vol. 2, No. 2, 1988 (pages 677-702), review appearance spectroscopy and survey some applications.
These and other works in appearance spectroscopy are based upon a simple spectrometer employing a tungsten filament which provides electrons which impinge on the sample to be studied. A grid electrically separates the filament and the detector assembly, and x-rays passing through the grid strike the walls of the chamber. The resulting photoelectrons are collected on a positive electrode. The resulting signal is amplified and synchronously detected by a phase-lock amplifier.
Previous work in appearance spectroscopy, however, has failed to consider a focused electron beam projected towards particular locations on the specimen, but instead found the properties of materials over large areas.
SUMMARY OF THE INVENTION
In accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method of and an apparatus for determining the chemical composition of a feature on a substrate, comprising the steps of directing a focused electron beam towards the feature, thereby causing the feature to emit x-rays, detecting x-rays emitted from the feature, while varying the energy of the beam and maintaining the focus of the beam on the feature, and determining the composition of the feature. The electron beam may be scanned over a surface of the substrate or stepped over the surface of the substrate to locations corresponding substantially to predetermined defect sites on the substrate.
The method may additionally comprise sequentially directing the focused electron beam towards each of a plurality of features on the substrate, sequentially detecting x-rays emitted from each of the features while varying the energy of the beam, and determining the composition of each of the features. The composition of the feature may be determined by monitoring the relative intensity of the x-rays while varying the energy of the beam. The energy may be gradually increased while the x-rays are detected. The composition of the feature can be determined from a lock-in signal corresponding to a derivative of the intensity of the x-rays. The use of lock-in detection as described herein includes, without limitation, those methods des
Hashmi Zia R.
KLA-Tencor Technologies
Lee John R.
LandOfFree
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