Radiant energy – Ionic separation or analysis – Methods
Reexamination Certificate
1998-08-24
2001-05-08
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
Methods
C250S309000
Reexamination Certificate
active
06229141
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved method for utilizing secondary ion mass spectrometry (SIMS) analysis in connection with insulator materials. More particularly, the invention involves an improved method for charge neutralization that can be used in connection with a SIMS analysis of insulators.
BACKGROUND OF THE INVENTION
Secondary ion mass spectrometry (SIMS) has become a purity analysis method of choice for surface and near surface investigation of solid samples because of its ability to provide parts-per-million to parts-per-billion sensitivity and excellent depth resolution. SIMS relates to a technique for surface and near surface analysis which involves ion bombardment of the sample surface for depth profiling. A good tutorial relating to the SIMS technique can be found on the Internet at http://www.cea.com/tutorial.htm, which is incorporated by reference in its entirety.
As discussed therein, SIMS involves the use of a bombarding primary ion beam onto a sample. Primary ions are implanted and mix with sample atoms to depths of 1 to 10 nm depending on the bombarding energy. The bombarding ion produces monatomic and polyatomic neutrals and ions of sample material, resputtered primary ions, as well as electrons and photons. Sputter rates depend on primary beam intensity, sample material, and crystal orientation. Sputter rates in typical SIMS analyses vary between 0.5 and 5 nm/s.
The primary ion beam species that are typically useful in SIMS analysis include Cs
+
, O
2+
, O
−
, Ar
+
, and Ga
+
at energies between 1 and 30 keV.
SIMS instruments include both static SIMS instruments, e.g., time-of-flight instruments, and dynamic SIMS instruments. Dynamic SIMS instruments in turn primarily employ use two kinds of mass analyzers, magnetic sector and quadrupole analyzers.
Magnetic sector instruments are common in the field. In these instruments, the ion beam passes through the magnetic field where the particles are acted on by a force at right angles, both to the direction of motion and to the direction of the magnetic field. Modern mass spectrometers use non-normal pole faces for entrance and exit of the ion beam to the magnetic sector. The fringings fields in this configuration compress the ion beam in the vertical direction (in and out of the screen) as it passes through the sector. Fewer ions strike metal surfaces and the ion beam focuses better at the exit slit with non-normal pole faces. The entrance and exit slits can be arranged at ion beam crossovers for the cleanest separation (highest mass resolution) between ions with similar m/z values.
Quadrupole mass analyzers, on the other hand, have been employed in many kinds of analysis since their invention in 1953. Quadrupole analysis employ rods that ideally have hyperbolic shapes, but this geometry can be approximated with closely spaced circular rods. In a typical quadrupole spectrometer, the rods are 1 cm in diameter and 20 cm long. Ions enter at a relatively low energy (~25 eV). Alternating and direct voltages on the rods cause the ions to oscillate after entering the quadrupole. For a given set of voltages, ions with a single mass-to-charge ratio undergo stable oscillation and traverse through the rods. All other ions have unstable oscillations and strike the rods. The alternating frequency and the ratio between the alternating and direct voltages remain constant.
The widespread use of SIMS technology can be seen, for example, from the article “Secondary Ion Mass Spectrometry—First Microelectronics, Now the Rest of the World”, by F. A. Stevie,
Surface and Interface Analysis
, Vol. 18, 81-86 (1992), which is incorporated by reference in its entirety.
The use of SIMS in connection with insulating materials is also known. However, it has also suffered from a number of problems. As shown in
FIG. 1
, ion bombardment
1
, of an insulating surface
2
, induces a positive charge
3
, onto the oxide surface. This problem has been addressed by a variety of approaches, the most common of which involves the use of an electron beam
4
, that seeks to neutralize the positive ion buildup. Electron beam neutralization has been used in connection with both magnetic sector and quadrupole based SIMS instruments.
Despite the effectiveness of electron beam neutralization in connection with quadrupole and magnetic sector instruments, there are nonetheless significant limitations on the use of SIMS in certain environments. One specific area in which SIMS analysis, and in particular magnetic sector based SIMS analysis, has not been effective involves the determination of alkali elements in insulators.
This problem is not unknown in the art and as was discussed, for example, in “Depth Profiling of Sodium in SiO
2
Films by Secondary Ion Spectrometry”, C. W. Magee et al.,
Applied Physics Letters
, 33, 193 (1978).
Traditional SIMS analysis of alkali metals in insulators has been deficient because the alkali elements will, in essence, “move” when an electric field is applied during SIMS testing. Such movement provides a false negative, i.e., will appear to indicate that the alkali metals are not present when in fact they are present in the insulator or, alternatively, will not show the proper location of the alkali elements.
The ability to accurately determine the presence of alkali elements as well as their quantity and exact location in insulators is particularly critical in the manufacture of semiconductor materials. The movement of alkali elements in the insulator under application of an electric field results in an insulator that acts as a conductor. Thus, insulators including alkali elements are largely useless in the field of electronic devices.
The primary technique that the art has employed in order to minimize this problem has involved the introduction of a conductive film coating onto the surface of the insulator. However, such a coating can introduce significant contamination problems during fabrication. Thus, it is an unacceptable solution to the problem of alkali metal movement in the field of semiconductor manufacture.
Accordingly, the need still exists for a method for accurately analyzing alkali elements in insulators.
SUMMARY OF THE INVENTION
Among other aspects, the present invention is based upon the surprising discovery that controlling the depth of electron penetration during charge neutralization in SIMS is effective in solving the problem of alkali element mobility and providing an accurate determination regarding the presence of alkali elements in an insulator.
Accordingly, the invention involves an improved secondary ion mass spectrometry (SIMS) analysis of an insulator that employs the use of an electron beam in charge neutralization. In particular, the method provides for controlling the electron penetration depth so as to at least substantially minimize the movement of alkali elements in the insulator.
In one aspect of the invention, the present invention relates to a method for charge neutralization in SIMS analysis using magnetic sector analysis. This method involves:
(a) providing a material having an insulator layer thereon;
(b) applying an ion beam onto a region of the surface of the insulator; and
(c) applying an electron beam onto at least a portion of the ion beam region, wherein the depth of electron penetration is not substantially greater than the depth of the dielectric layer.
This method can be used where the insulators is present on a substrate or where the insulator is present on another layer which itself is directly or indirectly on a substrate.
In another aspect, the claimed invention relates to a method for charge neutralization in SIMS analysis using a quadrupole instrument. This method comprises:
(a) providing an insulator material;
(b) applying an ion beam onto a region of the surface of the insulator material; and
(c) applying an electron beam onto at least a portion of the region such that the depth of electron penetration is not substantially greater than the depth of ion penetration.
The inventive method can be used in a variety of envir
McKinley Jennifer M.
Stevie Frederick A.
Berman Jack
Lucent Technologies - Inc.
LandOfFree
Analysis of alkali elements in insulators using secondary... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Analysis of alkali elements in insulators using secondary..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Analysis of alkali elements in insulators using secondary... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2494696