Chapter 13: Partial Pressure Analysis

Chapter 13: Partial Pressure Analysis Previously several types of sub-atmospheric pressure gauge were described. All of these gauges share one common feature: they report the total gas pressure. Partial pressure analyzers, in contrast, provide more detailed information about the gases that exist in vacuum systems following evacuation (the so-called "Residual gases"). The data provided by partial pressure analyzers can be qualitative (specifying the identity of the gases present), or quantitative (giving the partial pressure of each gas). As one might expect, instruments that can identify and measure the partial pressures of individual gases that exist in a working vacuum system are somewhat more complicated than simple total pressure gauges. Partial pressure analyzers, or residual gas analyzers (RGA's) as they are commonly known, function by ionizing samples of gas from the vacuum system, separating the ions into discrete groups based upon their masses, and then counting the amount of ions in each group. The details of each of these steps will be discussed in the unit. Partial pressure analysis is a comparatively recent addition to vacuum technology. While the principles of mass spectroscopy (analysis of ionized gases on the basis of mass differences) have been known since 1918, practical application in the field of vacuum technology was not demonstrated until 1960. In his ground-breaking work, H.L. Caswell used a mass spectrometer to show the beneficial effect of viton gaskets over other elastomer seals, and also the effectiveness of Meissner coils and getter pumps. Today we can select from a wide variety of partial pressure analysis instruments which conveniently attach to standard vacuum hardware. These instruments can range from small, simple to operate and reasonably inexpensive units used to monitor specific gases in a vacuum process chamber, to large, extremely sensitive and very expensive instruments used to detect minute traces of gases. Although there exist quite a few methods by which ions may be separated, only two of these methods are used in current commercial partial pressure analyzers: magnetic field and electric field separation. SPATIAL SEPARATION Electric fields Quadrupole Monopole Magnetic & Electric fields Cycloidial Cyclotron Resonance Magnetic fields Magnetic sector TEMPORAL SEPARATION Time of flight Page 152 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

Specifically, we will discuss the principles of operation of quadrupole and magnetic sector mass spectrometers. Both of these mass spectrometers fall in the category of "spatial separators" that is, they physically separate beams of ions on the basis of mass-to-charge ratio. Time of flight mass spectrometers, in contrast, rely upon the differing velocities of ions having different masses as a means of separation. The concept of mass separation was introduced in the previous unit on leak detection. Helium mass spectrometer leak detectors are, in fact, partial pressure analyzers (usually of the magnetic sector type) which are permanently tuned to detect a test gas such as helium. Ionization of Gas While other methods for ionization of sub-atmospheric pressure gases exist (such as field emission and chemical ionization), the most widely used technique for partial pressure analyzers is electron-impact ionization. Electrons emitted from a heated metal filament are electrostatically attracted to an anode, or electron collector plate, by an imposed electric field of from 50 to 150 V DC. On route to the anode, some of the electrons strike neutral gas molecules, stripping off one or more outer-shell electrons, creating positive ions. Some molecules may be split into fragments during this process, each fragment being a positive ion which will be mass separated and detected in the spectrometer. Filaments used in partial pressure analyzers may be made from a variety of refractory metals and alloys, each having unique characteristics that become important when performing critical work. Pure tungsten filaments when heated emit significant amounts of carbon monoxide and carbon dioxide. Iridium filaments which have been treated with thorium ("thoriated") are selected for use when high partial pressures of oxygen will be present. It should be noted that thoriated iridium filaments are susceptible to contamination from hydrocarbons. When this occurs, the electron emission from this type of filament will be degraded. Other special purpose filaments may be made of rhenium or lanthanum hexaboride. 50 to 150 V anode ionized molecules and fragments electron-emitting filament neutral gas molecules Figure 13.1 The components in the ionizer of a partial pressure analyzer. Page 153 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

Not all of the molecules which enter the ionizer exit the other side as ions. For a fixed electron accelerating potential (70 V, for example), the probability of ionization is gas specie dependent. Table 13.1 Ionization probability by electron impact ( 70 ev electrons) for common gases. Gas specie Ionization probability nitrogen 1.0 hydrogen 0.42 helium 0.14 methane 1.57 neon 0.22 carbon monoxide 1.07 nitrous oxide 1.25 oxygen 1.02 Argon 1.19 carbon dioxide 1.36 Krypton 1.81 In addition to having different ionization probabilities for a given electron energy, the response of each gas to electrons of differing energies is unique. Fortunately, the ion production by electron impact for each gas specie is directly proportional to the partial pressure of that gas specie. Acceleration of Ions Once positive ions are created in the ionizer, they are accelerated towards the mass separator by an electric field applied to a set of apertures called the accelerating aperture or entrance aperture. The degree to which ions are accelerated is a function of the mass of the ion, the charge on the ion, and the accelerating voltage (V a ) on the entrance aperture. For singly charged ions, accelerated by a fixed voltage, V a, the velocity to which the ions are accelerated is greatest for ions of low mass and lowest for ions of high mass. In some mass spectrometers the accelerating voltage is ramped from an initial low value to a higher value in order to aid in mass separation. Typical values for the bias on the acceleration apertures are from 1 to 5 kv DC. Mass Separation Of the two mass separation techniques that will be covered in this unit, (quadrupole electric field separation and magnetic sector separation), the magnetic sector is the easiest to understand. In this method, ions emerging from the ionizer and accelerated by the entrance slit enter a strong magnetic field (usually generated by a permanent magnet). Under the influence of this magnetic field the trajectory of the ions is bent according to the formula given in 13.1 below. Page 154 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

r = 1. 44x10 4 B MV a z where: r = radius of curvature of ion trajectory [m] B = magnetic field strength [tesla] M = molecular weight of ion [g / mole] V a = accelerating potential [volts] z = charge on the ion Sample Problems: 13.1 Calculate the radii of curvature for common atmospheric gases and water vapor using the following criteria: magnetic field strength = 0.1 Tesla, accelerating potential = 2000 V, all species are singly ionized. 13.2 Explain why it is the mass-to-charge ratio that determines the trajectory of an ion in a magnetic sector mass separator rather than simply the mass of the ion. ion source 1-5 kv slits r M-1 z M+1 z M z detector magnet Figure 13.2 Simplified drawing of the components in a magnetic sector mass spectrometer. As is suggested pictorially in figure 13.2, for a given set of conditions (accelerating potential and magnetic field strength) only ions of a specific mass-to-charge ratio will have the correct trajectory to pass through the slits just before the detector. Ions that have a larger mass-to-charge ratio are less strongly deflected by the magnet, and swing wide of the exit aperture. Similarly, ions with a lower mass-to-charge ratio have their trajectories more severely curved by the magnetic field, and also are prevented from reaching the detector by the exit aperture. Page 155 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

In practice, to achieve mass selection by varying acceleration potential alone would require a power supply capable of generating stable sweep voltages across a large voltage range. The practical solution to this problem is to divide the mass-to-charge range into two or three segments, and to use multiple permanent magnets to augment the magnetic field strength. In this method the atomic mass unit (AMU) range of from 2 to 50 is scanned using a 0.1 Tesla permanent magnet, while the 50 to 300 AMU range is scanned using a 0.25 Tesla magnet. In some expensive mass spectrometers electromagnets are used instead of permanent magnets. The electromagnets in these units have variable field strength, based upon the amount of electric current passed through the coils of the electromagnet. Quadrupole mass spectrometers use AC and DC electric fields to perform separation of ions based upon the mass-to-charge ratio. Figure 13.3 Simplified representation of the electrical circuits supplying AC and DC voltages to the two pair of rod-shaped electrodes in a quadrupole mass separator. As the name suggests, there are four "poles" or rod-shaped electrodes in a quadrupole mass spectrometer that function to separate ions based upon the mass-to-charge ratio of the ions. The poles of the spectrometer are paired electrically as shown in figure 13.3. One set of opposing electrodes are biased positively using a DC power supply, while the other two are biased negatively by another DC power supply. A radio frequency (RF) alternating current is superimposed on the DC voltage applied to both sets of electrodes. The rods are held in precise position with respect to one another and the other components of the spectrometer by precision machined ceramic discs. Each disc has four holes in it to support, align and electrically insulate the four rods. These ceramic supports allow the rods to be accurately repositioned in the spectrometer following cleaning. Page 156 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

The set of rods biased positively by a DC power supply acts as a "high-pass" filter collecting ions having a mass-to-charge ratio greater than a specified value. The Other set of rods, biased negatively, by the other DC power supply acts as a "low-pass" filter, and collects ions having mass-to-charge ratios less than a certain value. Together the two sets of rods provide an effective means for allowing only the ions having the desired mass-to-charge ratio to be counted at the detector. - + + - U +/- V +/- V U + V(cos ωt) +U U - V(cos ωt) Figure 13.4 A positive ion of low mass-to-charge ratio oscillating under the influence of applied AC and DC electric fields. As shown in figure 13.4 ions having a low value of mass-to-charge are strongly affected by the radio frequency AC current superimposed upon the positively biased rods. The amplitude of oscillation for these ions grows rapidly as the ion moves through the mass separator until the ion strikes one of the rods. Ions which impact a rod lose their charge and cannot be detected. Ions of high mass-to-charge ratio are "filtered' by the effect of the rods which have a negative DC bias as shown in figure 13.5. These more massive ions are much more sluggish in their response to the RF AC electric field than the lighter ions. The net effect of the negative bias on the more massive ions is to gradually drag them towards one of the negatively biased rods as the ion passes through the mass separator. Again, once an ion collides with an electrode, it loses its charge and cannot be detected. By choosing appropriate values for the acceleration potential, and the DC and AC bias potentials, a very effective mass filter can be created. In practice, one parameter (accelerating voltage, RF or DC potentials) is varied in time, and ion current is recorded for each mass-to-charge ratio. Page 157 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

- + + U - +/- V +/- V U + V(cos ωt) +U U - V(cos ωt) Figure 13.5 A positive ion of high mass-to-charge ratio oscillating under the influence of applied AC and DC electric fields is strongly attracted to the rods having a negativedc bias. Detection of Ions For either type of mass separator (magnetic sector or quadrupole) the ions which are not "selected out" impact the ion detector, where they generate an electrical signal. This signal is amplified electronically and sent on to pulse counting circuitry, and finally emerges as intensity (ion current) versus mass-to-charge ratio. Several types of ion detectors are used in commercial mass spectrometers. Simple, inexpensive units often employ a Faraday cup, while the more sensitive, higher-end units use either a Faraday cup/secondary electron multiplier combination or a channel electron multiplier. The sensitivity of ion detectors is typically specified in terms of electrical current per pressure, such as amps/torr. Values for the sensitivity of detectors can range from 4 x 10-6 to 1.0 x 10-5 Amp/Torr, assuming nitrogen ions. For an operational pressure range of from 10-2 to 10-12 Torr, the current range that a typical detector must be able to register is from 10-6 to 10-17 amps. Page 158 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

ions from mass-selector -V first dynode Faraday cup anode Figure 13.6.Ion detector for a mass spectrometer. This is a diagram of a combined Faraday Cup and secondary electron multiplier type detector. In a compound detector, as pictured in figure 13.6, the broad range of possible ion currents is handled by operating either the Faraday cup alone or in conjunction with the secondary electron multiplier (SEM). Ion currents from 10-6 to 10-12 amps may be measured using the Faraday cup alone. Ion currents of from 10-12 to 10-17 are measured by grounding the Faraday cup and applying a negative bias (-1 to -3 kv) to the resistor chain attached to the dynodes of the secondary electron multiplier. During operation of the SEM the initial low ion current (10-12 to 10-17 amps) is amplified by electron multiplication. Ions striking the grounded Faraday cup create secondary electrons upon impact. These secondary electrons are electrostatically attracted to the first dynode of the SEM. These dynodes are often fabricated from material which readily emits many electrons during bombardment with electrons. Copper-beryllium alloys (Cu 2-4 %, Be) which have been heat-treated to create a beryllium oxide surface film exhibit this favorable electron emission characteristic. Electrons created at the first dynode are attracted to the second dynode by the applied electric field, and upon striking the surface of the second dynode, again generates a cascade of secondary electrons for every arriving electron. In this manner, signal gains of from 10 5 to 10 6 may be achieved. Sample Problems: 13.2 Describe the differences in the principle of operation between magnetic sector and quadrupole mass spectrometers. Page 159 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

13.3 What is the operational pressure range for mass spectrometers? What will happen if a mass spectrometer is operated at pressures above the maximum suggested value? 13.4 Describe the operation of a combination Faraday cup / secondary electron multiplier detector in a mass spectrometer. 13.5 What characteristic of Cu-Be alloys make them a good choice for the dynodes of a secondary electron multiplier? Another type of ion detector is the channel electron detector. These detectors achieve gain by the same mechanism as the SEM previously described: electron multiplication. positive ions electrometer -HV Figure 13.7 Channel electron multiplier and associated electronics. In the channel electron multiplier, an ion incident upon the funnel shaped cathode creates a cascade of secondary electrons that are electrostatically attracted down the curved electron multiplier tube. The tube is made of a special glass containing lead oxide and bismuth oxides. The inherent high resistivity of the glass provides an electrical resistivity similar to that made by the chain of resistors in an SEM. The channel multiplier tube is curved for two reasons: it prevents positive ions from traveling backwards through the tube, and to maximize the effective number of "dynodes" for electron multiplication. An advantage of this type of electron multiplier over an SEM is that the channel electron multiplier can withstand repeated exposure to air. Both types of electron multipliers have a finite useful lifetime, which is generally on the order of one to two years. Be aware that stray magnetic fields (from ion pumps, for example) can affect the trajectory of electrons within either type of electron multiplier. The culmination of this complex series of steps (ionization, acceleration, mass selection and ion detection) is the representation of the data as signal intensity as a function of mass-to-charge ratio. Almost universally this data is output to a CRT screen as a graph which may look something like that presented in figure 13.8. Page 160 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

Hydrocarbon Oil Intensity 0 20 40 60 80 100 Mass-to-charge ratio Figure 13.8 Partial pressure analysis data taken from a vacuum vessel contaminated with hydrocarbon oil. Operation of Partial Pressure Analyzers Care should be taken in the use of partial pressure analyzers. Instruments of this type, even the "low-end" units are quite expensive and easily damaged by misuse. Installation of an analyzer on a vessel should be made with the following questions in mind: 1) What characteristic of the vacuum environment am I attempting to measure? (qualitative versus quantitative data). 2) What will the maximum pressure be in the spectrometer? 3) What mass range of gas (AMU) is expected? 4) Will the vessel and the spectrometer need to be baked-out? If so, at what temperature? 5) Is contamination of the partial pressure analyzer possible? How can the possibility of contamination be minimized? 6) Will the resolution of the partial pressure analyzer be sufficient for the application. Definition of the Peak Resolving Ability of a Mass Spectrometer Resolution in a mass spectrometer may be broadly defined as the ability of the instrument to clearly identify signals from ions of two similar mass-to-charge ratios. There are several accepted means for analytically defining the resolution of a mass spectrometer. For adjacent peaks M 1 and M 2 in a spectra, if the intensity in the valley (h) between the peaks is less than 10% of the value of the intensity at peak maxima (H) the resolution is defined as M 1 / (M 1 and M 2 ) (see figure 13.9). Page 161 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

R = M 1 M 2 M 1 H for H > 10 h h M 1 M 2 Figure 13.9 Definition of resolution for two adjacent peaks observed in a mass spectra. For a single peak in a mass spectra, the resolution may be defined as the quotient of the mass-to-charge ratio at maximum peak intensity divided by of peak width at half maximum intensity, as shown in figure 13.10. H M H/2 R = M M M Figure 13.10 Definition of resolution for a single peak observed in a mass spectra. Differentially Pumped Partial Pressure Analyzers Some vacuum processes are conducted at pressures above the recommended value for operation of partial pressure analyzers. Examples of such processes include: sputter deposition and plasma etching. It may be very useful to diagnose processes such as these using partial pressure analysis. This is typically done by limiting the flow of process gases into the spectrometer and by adding a dedicated high vacuum pump to evacuate the spectrometer. Such a system is referred to as a "differentially pumped" partial pressure analyzer. A drawing of such an instrument is presented in figure 13.11 The purpose of the aperture between the analyzer and the vessel is to limit flow of gas through the spectrometer. The second (often variable opening) aperture's function is to allow control of the pumping speed of the high vacuum pump dedicated to the Page 162 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

spectrometer. Fixed apertures may conveniently be made by drilling a small hole in a copper disc which is then substituted for the copper gasket in the flange joint. Small turbomolecular pumps are often selected for this application, as they generally produce very little contamination due to backstreaming, do not create a strong magnetic field (as do ion pumps). Differentially pumped partial pressure analyzers find much use in the semiconductor industry. It is of much economic importance to that industry to have the capability to accurately determine the "endpoint" of a plasma etching process. This is accomplished by monitoring the partial pressures of the gaseous by-products of the etching process. Similarly, in the process of thin film deposition by sputtering, it is occasionally very useful to monitor the purity of the process gas and any contaminants due to outgassing, permeation or leaks. Sample Problem: 13.6 Calculate the resolution of a mass spectrometer if the width of a peak at half maximum is 0.1 AMU for a peak centered at 50 AMU. Vacuum vessel RGA aperture 2 aperture 1 auxilliary pump Figure 13.11 Cross-section of a differentially pumped partial pressure analyzer with two flow limiting apertures and an isolation valve. Residual Gases in Vacuum Vessels - Their Characteristics and Probable Sources Quite often the technique of partial pressure analysis is applied to a vacuum system which is exhibiting out of normal performance (high base pressure, frequent filament burnout for systems with heated filaments, poor film adhesion in deposition systems, etc.). Interpretation of data from a partial pressure analyzer can be made significantly more straight-forward if some information about the recent history of the vacuum system under study is known. As with most fault-finding techniques is it often useful to start with the most recent occurrences (vessel modifications, significant deviations from normal operating procedures, etc.) and work backwards. Below are some questions which may provide insight into the sources of residual gases in a vacuum vessel. 1) Has any fixturing internal to the vacuum vessel been modified or replaced. 2) Has the system been baked out recently? Page 163 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

3) What is the condition of the high vacuum pump stack (including the cold trap, if applicable)? Reference Information for Partial Pressure Analysis Assuming a partial pressure analysis is performed on a system, the first step in turning the data into meaningful information is to gain a qualitative understanding of the nature of the gases in the system. As an aid in this process table 13.2 provides some information relevant to specific mass-to-charge ratios which may show peaks of varying intensity in a partial pressure analysis. Mass-tocharge ratio Suspected gas specie Comments 2 hydrogen hydrogen is often the major gas load in UHV systems due to permeation through stainless steel vessel walls. Dissociation of water and hydrocarbons may also give a peak at 2. 4 helium May be present following leak checking. Helium also permeates elastomeric seals. 16 oxygen Singly ionized monatomic oxygen may be present due to dissociation of water, or from an air leak. 18 water In the high vacuum range water vapor is the largest contributor to the gas load. If water is present peaks should also be seen at 16 and 17. 19 fluorine May indicate the decomposition of fluorinated hydrocarbons in the vessel. 20 neon May be observed in UHV systems with ion pumps. 28 nitrogen Diatomic nitrogen, single ionized. If nitrogen is present, an air leak may be the cause. A peak at 14 for monatomic nitrogen, singly ionized should also be present. 28 carbon monoxide Resistively heated tungsten filaments emit significant amounts of CO. Turn off ion gauges to reduce this effect. Peaks for carbon and oxygen should be present. Page 164 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

30 nitrous oxide Another possible by-product from heated filaments within the vessel. 30 ethane (C 2 H 6 ) The presence of ethane in a vacuum vessel suggests hydrocarbon contamination. 32 oxygen Diatomic oxygen, singly ionized. The presence of this specie may indicate an air leak, especially if a stronger peak at 28 (nitrogen) is observed. 40 Argon Argon may be present due to an air leak. Check for oxygen and nitrogen. 44 carbon dioxide May be generated from heated tungsten filaments, as with CO. 45 isopropyl alcohol May be a residue from a cleaning process used on a component in the vessel, especially in tapped holes. 58 acetone See comments for isopropyl alcohol. 95 trichloro-ethylene See comments for isopropyl alcohol. Another aid in gaining useful information from partial pressure data are the reference library and spectra search functions that are available on many modern computer-based instruments. With these features one may compare spectra obtained by the instrument to known reference spectra that exist in the library. Some of the correlation functions also provide the means to analytically describe the quality of the match between the data and the reference spectra. Some of the computer controlled instruments will also permit automated periodic sampling and will generate a history of the partial pressures of selected gases as a function of time. Understanding how to interpret scans from a mass spectrometer is a valuable skill. Use of the process of elimination will quickly provide a very short list of possible gas identities. The following simple rules will help in establishing which gases are likely present in the vacuum system under analysis: 1) Start with the most intense peak in the spectra. Assume that this peak is due to a singly ionized atom or molecule. Refer to figure 13.12 for this example. 2) Note the mass-to-charge ratio of this most intense peak. The molecular weight (if the single ionization assumption is correct) cannot be more than the value of the mass-to-charge ratio for this peak. Page 165 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

{The most intense peak occurs at mass-to-charge ratio 18. By our assumption of single ionization, all elements of the periodic table having atomic weights greater than 18 are eliminated.} 3) Refer to the periodic table, using the listed atomic weights, write down the possible combinations of elements that have atomic weights that sum to equal the mass-to-charge ratio of the most intense peak in the scan. In our example, the mass-to-charge ratio is 18. the possible combinations of elements whose atomic weights sum to 18 are: 2H + O; N + He; 2Be, B + 2He; and C + He +2H. Of these, the only likely possibility is 2H + O, otherwise known as H 2 O. 4) Look at the peaks associated with the major peak which have lower mass-tocharge ratios. Determine if it is possible that these peaks may be molecular fragments of the major peak. In this example, some of the H 2 O has been dissociated in the ionizer of the partial pressure analyzer to create the fragments OH and O, which have mass-to-charge ratios of 17 and 16 respectively. 5) Mark those peaks that have tentatively been identified, and repeat steps 1 through 5 for the remaining peaks in the spectra. 100 90 80 70 Intensity 60 50 40 30 20 10 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Mass-to-charge ratio Figure 13.12 Data from the partial pressure analysis of a vacuum vessel containing a gas load primarily due to water vapor. Sample Problem: 13.7 List at least four practical applications for partial pressure analyzers. As was mentioned in the section describing the ionizer, molecular gases, such as water, carbon dioxide, and oil vapors will almost certainly become dissociated (fragmented) during the ionization process. Each of the fragments will become positive ions which will be accelerated, mass analyzed and detected. The peaks on a mass spectra that are due to the fragments of a disassociated molecule are often referred to as a "cracking pattern". Understanding this concept will aid greatly in both qualitative and quantitative interpretation of mass spectra In the following table are listed the fragments and massto-charge ratios for several common gases. Page 166 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

CH 4 CH 4+ + CH 3+ + CH 2+ + CH + + C + + H 2+ + H + (16) (15) (14) (13) (12) (2) (1) 13.2 H 2 O H 2 O + + HO + + O + +H 2 + + H + (18) (17) (16) (2) (1) CO CO + +C + + O + (28) (12) (16) Sample Problem: 13.7 The mass spectra for each of the cracking patters listed in equation set 13.2 is provided in figures 13.12 through 13.14. For each of these mass spectra, identify the peaks by writing the ion next to the peak it corresponds to. Methane Relative Intensity 100 90 80 70 60 50 40 30 20 10 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Mass-to-charge ratio Figure 13.13 Mass spectra of methane. Page 167 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

Carbon Monoxide Relative Intensity 100 90 80 70 60 50 40 30 20 10 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Mass-to-charge ratio Figure 13.14 Mass spectra of carbon monoxide. Further complication in the mass spectra is due to multiple ionization of gas species. H 2 O H 2 O + + HO + + O + +H 2 + + H + + H 2 O ++ + HO ++ + O ++ (18) (17) (16) (2) (1) (9) (8.5) (8) CO CO + +C + + O + + CO ++ + O ++ + C ++ (28) (12) (16) (14) (8) (6) High resolution mass spectrometers can also discriminate between the isotopes of gas species. An isotope of an element has a different number of neutrons in its nucleus than other isotopes of the same element. This difference in nuclear structure creates a slightly different atomic weight. This effect can be seen in the mass spectra of the noble gas, Argon(see equation 13.3, and figure 13.15) Ar 40 Ar + + 38 Ar + + 36 Ar + + 40 Ar ++ + 38 Ar ++ + 36 Ar ++ Page 168 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

Isotopes of Argon 100 Log of Relative Intensity 10 1 0.1 0.01 0 5 10 15 20 25 30 35 40 45 Mass-to-charge ratio Figure 13.15 Mass spectra of Argon, showing peaks for several isotopes Quantitative Analysis of Mass Spectra As was suggested earlier in this unit, two types of information about the residual gases in an evacuated vessel may be gained through partial pressure analysis: identification of species present (qualitative information) and the amount of each species (quantitative information). Inexpensive mass spectrometers typically do not have the resolving power required to clearly identify overlapping peaks (carbon monoxide and nitrogen, for example) and are typically not used for quantitative analysis of mixtures of gases. For spectra of mixtures of gases which do not have overlapping peaks, one may use the following steps to perform a rough quantitative measurement: 1) Identify all of the peaks in the mass spectra. 2) For each peak obtain from the instrument's manual the sensitivity of the instrument for each gas specie (S), as well as the detector gain for each specie (G). 3) Calculate the partial pressure of each gas using the formula provided in equation 13.5. P 1 = total ion current for peak 1 G 1 xs 1 where: P 1 = partial pressure of gas specie 1 G 1 = detector gain of gas specie 1 S 1 = mass analyzer sensitivity of gas specie 1 Analysis of mass spectra of gas mixtures in which peaks overlap, such as carbon monoxide and nitrogen are somewhat more complicated. The measured intensity of a peak will be the algebraic sum of the intensities of the two peaks which are overlapping. Page 169 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

One must use reference data to establish the ratios of peak intensities for the peaks from each of the gases in the mixture. Using the subsidiary peaks which do not overlap, estimate the partial pressures of the gases in the mixture, using the technique just described. Once this initial estimate of the partial pressures of each gas is in hand, use the gain and sensitivity to solve for the total ion current for each component that contributes to the intensity of an overlapped peak. Laboratory Exercise 13.1: Qualitative identification of species in mass spectra. Equipment required: none. Procedure: Using the table of cracking patterns for materials commonly used in vacuum technology (Appendix X), identify the constituents in each of the following mass spectra Laboratory Exercise 13.2: Operation of a Partial Pressure Analyzer Equipment required: small vacuum vessel or bell jar vacuum system capable of attaining a pressure of 10-5 Torr or lower; a complete partial pressure analysis instrument; calibrated leaks (helium, nitrogen, Argon). Procedure: Review the installation and operating guidelines for the partial pressure analyzer you have selected for this experiment. After reading and understanding the procedures in these instructions, inform the laboratory instructor of your procedure for installation and operation of the instrument. With his approval, begin the installation of the spectrometer head onto the vacuum vessel. Attach the mass spectrometer head to the vacuum vessel with an isolation valve between the two. Attach the reference leaks to the vessel. Connect the spectrometer to the control unit as suggested by the manufacturer. Evacuate the vacuum vessel to a pressure of 10-5 Torr or less. Following the manufacturer's operating procedures, obtain the partial pressure analysis of the residual gases in the vessel. Repeat the measurement at five minute intervals for an hour to see how the partial pressures of gases in the vessel change during operation of the high vacuum pump. Following this series of measurements, open one reference leak briefly (1-2 seconds), and observe the mass spectra. Note any changes in the mass spectra. Wait until the mass spectra returns to a "baseline" reading similar to that prior to the injection of gas from the reference leak. Repeat the controlled injection of known gas with the remaining reference leaks that are attached to the vessel. Following completion of all experimental work, turn off the partial pressure analyzer following the manufacturer's suggested procedures. Shut down the vacuum system safely, and vent the pumps and vessel. Write a laboratory report of your procedures and findings, including the data collected. Page 170 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002