Method of and apparatus for measuring trace amounts of oxygen.

This invention relates to means for measuring trace amounts of oxygen, and more particularly to apparatus and method for monitoring trace amounts of oxygen especially suitable in semiconductor fabricating processes.

In the process of manufacturing semiconductor device and gaseous plasma display panels the presence of trace levels of oxygen either in the process environment or on the device surface could cause product failure either as a process yield or product reliability problem. For example, during chip joining a "thin skin" of oxide on the joining surfaces could cause bondability problems. As another example, trace levels (10 ppb to 100 ppm 0₂) in the gas display panel can cause a failure in the plasma discharge. These examples demonstrate the need for a sensitive trace oxygen analyzer which can operate in the dynamic range of 1 part per billion (ppb) up to the percent level. Commercially available trace oxygen detectors which are on the market today have a lower limit of 0.1 part per million.

Here the invention is to provide an improvement.

Thus the object of the invention is to present a novel method and apparatus for measuring trace amounts of oxygen over a wide dynamic range extending from parts per billion (ppb) to percent amounts of oxygen and to provide means for monitoring or measuring trace amounts or trace levels of oxygen in semiconductor device fabrication processes or levels adsorbed on device surfaces.

The object of the invention as it is characterized by the attached claims is achieved by the formation (from the trace oxygen and controlled water) of a preferred O(H₂O)₂ ion of m/e (mass to charge ratio) of 52 formed at atmospheric pressure which is measured and related through a calibration curve to the concentration of oxygen.

The present invention has a particular advantage in the detection of low level 0₂ contaminants in gases or oxygen absorbed in various solid surfaces, e.g. wafers.

A preferred embodiment of the invention will now be described in detail in conjunction with the accompanying drawings. <ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic of a plasma chromatograph mass spectrometer adapted for use in this invention.</li><li>Fig. 2 is a graph illustrating the peaks in the negative ion mode spectrum of m/e (mass to charge ratio) of 52 and 32 recorded for a predetermined period of time.</li><li>Fig. 3 illustrates the peak intensities of ions graphed as a function of time.</li></ul>

The trace oxygen detector system described in this application can detect oxygen at the part per billion (ppb) level, with the advantage that it can detect oxygen that is present in the gas phase as well as oxygen that is adsorbed on a surface. Also by extending the method of this invention, the bond strength of the adsorbed oxygen to the surface can be determined. The basis of the technique of this invention, is the formation (from the trace oxygen and controlled water) of a preferred O(H₂O)_<o>2</o> ion of m/e (mass to charge ratio) of 52 formed at atmospheric pressure which is measured and related through a calibration curve to the concentration of oxygen. The formation of the O(H₂0)2 ion will be described below.

Referring to Fig. 1 the instrument or apparatus comprises an atmospheric pressure ionization source, a reaction region, and a quadrupole mass spectrometer. The ionization source utilizes a ⁶³Ni foil for initiating the ionization process. Ultra high purity nitrogen N₂, is used as the carrier gas to bring the oxygen into the instrument. The method used to sample the trace oxygen and mix it with the carrier gas depends upon the state the oxygen exists in (gases or adsorbed film). High purity nitrogen with 10 ppm water is used as the flush gas to flush out the ionization source and also supply the source of water for the clustering reaction with oxygen and as such can also be considered as the reagent gas. The ionization source is operated at atmospheric pressure with a small electric field of approximately 200 V/cm being applied across the ionization source and the length of the reaction region. The field is biased in such a way as to repel the negative ions down through the reaction region towards a pinhole aperture which separates the reaction region from the quadrupole mass filter section of the instrument. As the ions pass through the pinhole aperture (e.g. about 1 mil diameter) they are focused onto the quadrupole mass filter by an ion lens focusing arrangement. The quadrupole mass filter is tuned, in its context, to respond only to m/e values of 32, 50, 52, and 68 which correspond to <o>O</o>₂ , O₂(H₂O)<o></o>, O(H₂O)_<o>2</o> and and 0₂(^H₂0)₂, respectively. An electron multiplier is mounted on the axis and serves as the detector for these ions. The specific mass filter section of this instrument operates at approximately 4 x 10-8 µbar. A schematic diagram of this instrument is shown in Fig. 1.

The chemical reactions which take place in the ionization source at atmospheric pressure are very complex. The basic functional and preferred ion to be employed in this invention is the O(H₂0)₂ because it is very stable under analysis conditions. The 0₂ molecular oxygen ion, which is the most familiar ion form of oxygen, is unstable and undergoes charge transfer reaction with other molecules. Therefore, the 0_2- ion is undesirable in some applications. To keep the 0₂ ion from forming in the ionization source, the temperature of the ionization source is maintained above 483 _K. At 483 K the coefficient of thermal detachment of an electron from the 0₂ ion is greater than the coefficient of electron attachment. <chemistry id="chem0001" num="0001"><img file="EP0032984A2_D0001.tif" /></chemistry>As shown in reaction (1) when an electron is captured by oxygen it forms a highly energetically excited ion and at temperatures above 483 K there is enough thermal energy to cause thermal electron detachment as shown in reaction (2). In other words, at temperatures above 483 K the rate of reaction (2) is greater than the rate of reaction (1).

Therefore, at or above 483 K the predominant mode of ionization for oxygen is by dissociative electron capture as shown in reaction (3). <chemistry id="chem0002" num="0002"><img file="EP0032984A2_D0002.tif" /></chemistry><chemistry id="chem0003" num="0003"><img file="EP0032984A2_D0003.tif" /></chemistry>

The atomic oxygen ion will, under a hydration reaction in the presence of water, form the stable O(H₂O)_<o>2</o> ion as shown in reaction (4). It is this ion which is monitored as a function of the oxygen concentration.

As the oxygen concentration of the gas to be measured increases into the percent range a side reaction starts to compete with reaction 2 as shown in reaction (5) <chemistry id="chem0004" num="0004"><img file="EP0032984A2_D0004.tif" /></chemistry>

In reaction (5) molecular oxygen 0_2' reacts with the excited molecular oxygen ion O₂<o></o>, and adsorbs the excess energy and thus stabilizes the 0₂ ion. A peak at m/e of 32 is noted in the mass spectrum. This ion can be used at higher (percent) concentration as a monitor of the oxygen concentration.

A significant advantage of this instrument or method is its wide dynamic range of operation; that is, parts per billion (ppb) to percent concentration of oxygen. Using this method/instrument it is possible to detect instantaneous changes in oxygen concentrations and therefore it can be used as a continuous oxygen monitor. The method/instrument is capable of being easily automated with any microprocessor or minicomputer such as an IBM Model 5100 or IBM Model 5110.

Another significant advantage is that no or very little sample preparation is needed (primarily bacause there is no vacuum associated with the sample housing).

Best Mode For Carrying Out The Invention

The Alpha-II plasma chromatograph/mass spectrometer manufactured by Franklin GNO Corporation was adapted for use in this invention. This instrument consists of a Beta-VII plasma chromatograph coupled to a specially modified Extranuclear Laboratories Spectr-E1 quadrupole mass spectrometer. The operating parameters of the plasma chromatograph used in this study are summarized in Table I. <tables id="tabl0001" num="0001"><img file="EP0032984A2_D0005.tif" /></tables>

A schematic of the combined PC-MS system is shown in Fig. 1. This instrument has the capability of operating in several modes so that ion mobility spectra, mass spectra, total ion mobility spectra, and mass identified mobility spectra can be obtained. The ionic mobility may be measured in either a one-grid or two-grid pulsing procedure with the Beta-VII plasma chromatograph mode of operation. Both grids of the drift tube are held open, allowing all the ions produced in the ionization source to continually drift down the tube and into the quadrupole mass spectrometer, which results in atmospheric pressure ionization mass spectra. Total ion mobility spectral can be obtained by operating the plasma chromatograph in the normal one-grid pulsing mode and the quadrupole mass spectrometer in the total ion mode, which enables the channeltron electron multiplier detector to measure the ionic distrubution as a function of time. By adjusting the mass analyzer to respond only to a single m/4 value and operating the plasma chromatograph in the normal one-grid pulsing mode, the distribution of an individual ion as a function of time can be obtained. The arrival time of the individual ions can then be compared with the arrival time of the ions measured in the total ion mode to produce the mass-identified mobility spectra.

A Nicolet Model SD-72IA integrating ADC (analog-digital converter) mounted in a Nicolet Model 1074, 4096-channel signal averager was used to digitize the accumulated plasmagrams. Usually 512 scans of 27 ms duration were collected and stored on magnetic tape with a Nicolet Model NIC-28A magnetic tape coupler and Kennedy Model 9700 tape deck. As the data was being accumulated in memory, the information was displayed on a Tektronix Model D10 oscilloscope. The data stored on the magnetic tape was entered into an IBM System/370 computer and was subsequently analyzed using the VSAPL program under the operating system VM/370. The position and intensity of the peaks in the mobility spectra along with a plot of the mobility spectra were then displayed on a graphics terminal from which hard copies could be obtained.

The instrument consists of an atmospheric pressure ionization source, a reaction region, and a quadrupole mass instrument. The ionization source utilizes a ⁶³_Ni foil for initiating the ionization process. 6³Ni emits a 60 keV B ray which is the source of electrons used for the formation of the O(H₂0)₂. Ultra high purity grade nitrogen, N₂, is used as the carrier gas to bring the oxygen containing gas into the instrument. The method used to sample the trace oxygen and mix it with the carrier gas depends upon the state in which the oxygen exist (namely a gas or absorbed on a surface). High purity nitrogen with 10 ppm water is used as the flush gas to flush out the ionization source and also to supply the source of water for the clustering reaction with the atomic oxygen ion and, as such, can also be considered as the reagent gas. The ionization source is operated at atmospheric pressure and at 210⁰C (483 K) or above. A small electric flield of approximately 200 v/cm is applied across the ionization source and the length of the reaction region and is biased in such a way as to repel the negatively charged ions down through the reaction region towards a pinhole aperture which separates the reaction region from the quadrupole mass spectrometer section of the instrument. As the ions pass through the pinhole aperture they are focused into the quadrupole mass filter by an ion lens focusing arrangement. An electron multiplier is mounted on axis and serves as the detector for the ions. The mass spectrometer section of this instrument operates at approximately 4 x 10^-8 ubar pressure.

Oxygen concentration in a gas: <ul id="ul0002" list-style="none"><li>To determine the oxygen concentration of a gas, a known volume of the gas is mixed with the ultra high purity nitrogen carrier gas and allowed to enter the ionization • source of the instrument. The peaks in the negative mode mass spectrum of m/e (mass to charge ratio) of 52 and 32 corresponding to the O(H₂0)₂ and 0₂ ions are then recorded for a predetermined amount of time (Fig. 2 shows a typical negative mode mass spectrum). The peak intensities for the ions are then graphed as a function of time; as in Fig. 3. The area under this curve is then integrated and this value is compared to a calibration curve to determine the concentration of oxygen.</li></ul>

The calibration curve is derived from similar measurements made on gases containing known concentrations of oxygen. Spectra are obtained over a predetermined amount of time, intensities of the m/e = 52 peak are plotted as a function of time and the area under the curve is measured. The calibration curve consists of these area values graphed as a function of the known oxygen concentration.

Oxygen concentration as an adsorbed layer on a surface: <ul id="ul0003" list-style="none"><li>To determine the amount of oxygen that is adsorbed on a surface, the surface to be investigated must be placed in a container of known volume. The container must then be flushed out with nitrogen gas to remove the oxygen from the gas phase as a result of air. A flush of 30 min, to one hour is more than sufficient for this purpose. Next the container must be sealed off and heated to degas the adsorbed oxygen from the surface. The oxygen is then measured as described above for the gaseous phase.</li></ul>

The system of this invention is characterized by the use of atmospheric pressure ionization using ⁶³Ni; a negative ion made of operation; specific ion detection e.g. m/e 32 [O₂<o></o>], 50 [O₂(H₂O)<o></o>], the optimum and preferred ₅₂ [O(H₂O)₂<o></o>], and ₆₈ [₀₂(_H20)2]; ultra high purity N₂ as a carrier gas; ultra high purity N₂ with 10 ppm H₂0 as a reagent gas; an ionization source temperature of at least 210°C; and an electric field strength of 200 V/cm.

The instrument of this invention differs from conventional plasma chromatograph-mass spectrometers in that no gating or scan grids are employed and therefore there is no drift spectrometer and no ion mobility spectrum. The instrument of this invention also differs from an API (atmospheric pressure ionization) mass spectrometer in two ways: 1, a reagent gas is used in addition to a carrier gas and 2, the use of a reaction region with an applied electric field biased along it.

One unobvious feature is the use of the negatively charged oxygen water clusters to monitor the amount of oxygen. A particularly unobvious feature is the operation of the ionization source at or above 210°C. This feature is of prime importance to form the O(H₂0)₂ ion. This ion of m/e value of 52 is the preferred and optimum ion to be monitored at extremely low oxygen levels (sub picogram level, sub ppb). The ions of m/e 32, 50, and 68 can be used to monitor the oxygen level at high concentrations (ppm). This instrument has the advantage that there is no vacuum associated with the sample inlet assembly and therefore there is very little sample preparation. Another significant advantage of this instrument is its wide dynamic range of operation e.g. ppb. to percent amount of oxygen.

This instrument will also respond to instantaneous changes in oxygen concentrations and therefore can be used as a continuous oxygen monitor. The instrument is capable of being easily automated with any microprocessor such as an IBM Model 5100.

While the invention has been illustrated and described with reference to preferred embodiments of this invention, it is to be understood that the invention is not to be limited to the precise constructions herein disclosed and the right is reserved to all changes and modifications coming within the scope of the invention as defined in the appended claims.