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A new type of transducer for partial pressure control
Surface and Coatings Technology, 36 (1988) 939 -947
939
A NEW TYPE OF TRANSDUCER FOR PARTIAL PRESSURE CONTROL* C. A. GOGOL and R. MUELLER Leybold Inficon Inc., 6500 Fly Road, East Syracuse, NY 13057 (U.S.A.) (Received April 12, 1988)
Summary A new transducer for controlling the partial pressure of sputtering gases is described. Utilizing the electron impact emission spectroscopy (EIES) principle, an instrument capable of operating in a total pressure environment of 100 X iO~Torr or more with minimum detectable partial pressures to 10’ Torr is described. These capabilities seem ideal for controlling reactive deposition processes utilizing N2, 02 or hydrocarbons. Because of the high operating pressure capabilities, this transducer does not require the complication of high vacuum pressure reduction and thus eliminates a major source of unscheduled maintenance. The operating principles of EIES and the sensitivities that were determined for the various gases tested are described in this paper. The present authors have found the technique to have high stability over extended periods of time (about ±3.5% over 30 days) and sufficient sensitivity to control processes of present practical interest. In addition to the use for direct partial pressure control the technique has utility for monitoring common residual gases such as H20 and CO2. in an analyzer mode, as a precursor to starting the reactive process.
1. Introduction Over the past several years the present authors have been developing a new technology for partial pressure analysis and control. This technology, based on electron impact emission spectroscopy [1, 2] has a set of unique properties and advantages for the intended applications, e.g. partial pressure control of reactive high rate sputtering [3] and low vacuum gas analysis. Specifically, the advantages are perceived to be as follows. (A) Stability: a long-term constancy far exceeding that demonstrated by other instruments used today. (B) Simplicity: a design that requires no moving parts, no critical voltages and no close tolerances. These all combine to make for an instru*paper presented at the 15th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 11 - 15, 1988. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
940
ment that has inherent long life and is relatively free from deterioration due to common contamination. (C) High operating pressure: the present implementation allows pressures up to 100 X iO~ Torr to be directly monitored without pressure conversion. (D) Wide dynamic range: a practical, useful range of up to 6 orders of magnitude, covering the minimum sensitivity (about 1 X 10~Torr for some gases) to the maximum operating pressure. (E) Quick response: the combination of good signal-to-noise ratios and unrestricted mixing of gases from the chamber. (F) Ease of maintenance: because a pressure reduction system is not needed and the instrument utilizes simple structures and subsystems, maintenance needs are minimal and performed by technicians with general skills. (G) Specificity for a wide range of gases: with the simple exchange of thin film optical filters a wide range of common gases may be monitored or controlled. These include argon N2, H20, 02, CO, CO2 and some hydrocarbons such as C2H2 and CH4. With these key points in mind, the instrument’s operation, performance and application are described.
2. Transducer description The operation of this transducer is based on the energy exchange between a low current (500 ptA), medium voltage (100 V) primary electron beam and the gas molecules or atoms in the vacuum chamber. Gas molecules that receive energy from a close encounter with a primary electron relieve that energy by emitting a photon equal in energy to the originally transferred energy. Since the allowed transitions are energy discrete, based on the electron configuration of the specific species, the photons produced are therefore frequency discrete and attributable to a specific gas species. Most gases have several transitions that are excited with high probability and will have a signature of wavelengths that allow unique identification even when the preferred transitions of several species occur very close in frequency. Together with the ability to identify a gas species by its photon frequency signature the ability to deduce its density is available through the measurement3)ofdue the tophoton flux. In simplistic terms, from the total the specific electron transition statephoton i to k flux J,,. (s’ cm number density N (cm3) of a specific gas species by the is related to the following:
=N(i/e)uk~
(1) where i (A/cm2) is the primary electron current, e (A s) the coulombic charge and Uki (cm2) the cross-section for exciting the gas atom from state k to i with the specific constant energy of the primary electron. The implica~ik
941
tion of this simple relationship is that, if the primary electron voltage and current are held constant, the total number of photons produced is directly related to the number density of the species. Thus at constant temperature the photon flux at a specific wavelength may be thought to be directly proportional to the partial pressure of a particular gas species. For this reason, we have chosen to call this instrument an optical gas controller (OGC). Of course it is not quite so simple in practice. While eqn. (1) might hold for the case of a very dilute gas, operation at higher pressure means that the effects of pressure broadening and self-absorption will be noticeable for many gases [4]. In addition, the general environment may consist of several gases simultaneously. At the pressures encountered, especially near the upper operating range, chemical interactions between the gases and, perhaps more importantly, multiple collisions of the primary electrons with gas molecules may significantly alter the output. Nevertheless, all the studied cases are at least monotonic with pressure. In the cases of primary interest, algorithms have been developed that take the chemical and pressure effects into account in a sufficiently accurate manner. These algorithms are series expansions, based on experimental observation. Once photons are produced, a portion passes through a vacuum-sealed window and is then frequency selected by a thin film filter. These “species”specific photons are converted to an electric current by a compact photomultiplier tube (PMT). Subsequent current-to-voltage conversion, demodulation and amplification once properly calibrated and linearized produce a display indication of partial pressure p as follows: PK(~Ta,Nb,,1’)k’(hik
(2)
where K(Na, Nb, P) is a normalization function of the densities Na, Nb, of the various species and total pressure P. k’ is a gauge factor that takes into account all the various photon collection and conversion efficiencies and is established at the time of calibration. The normalization function is used to provide reasonably accurate quantitative results for a specific situation. Qualitative information and trend analysis of gases is of course possible without knowledge of the normalization function. ...,
3. Implementation The transducer described above has been implemented in hardware as shown in Fig. 1. The basic exciting unit is built around the standard 2.75 in metal seal sexless vacuum flange. In this way the gases from the chamber have direct contact with the excitation volume. The 1.375 in opening offers minimum impedance to the diffusion flow of the gases to the excitation volume. As a consequence, delays and selectivities caused by a sampling system are avoided.
942
PMT
LS
:: ~
OF
/
M
__________
~-
______
Fig. 1. The basic OGC transducer. An electron beam (EB) from the hot filament (F) of the emitter (E) passes through the excitation volume (EV) where energy is exchanged
with gas molecules. Subsequent de-excitation produces photons (P) specific in wavelength for each species. A portion of the photons pass through the vacuum-sealed quartz window (W) through a bandpass optical filter (OF) and are converted into an electrical current in a photomultiplier (PMT). A filter and photomultiplier combination is used for each channel. Other features of the transducer are a thermal shield (TS) for fast warm-ups, a Faraday trap (FT) for the suppression of secondary electrons and a light shield (LS) to reduce unwanted radiation at the PMTs. The transducer is built around a standard 2.75 in vacuum flange (VF) and has provision for an auxiliary pump port (PP) for further pressure reduction. Filters can be exchanged by powering a lead screw with motor (M).
The OGC is a density-sensitive transducer. Because the apparent gas density can change as a function of temperature, a low thermal mass shield is introduced to reduce the time it takes for the filament to warm the structures that come in contact with gas. In this manner even batch processing situations can be handled without temperature-induced sensitivity drift. Hammatsu R1414 PMTs were used; they provide adequate signal gain and also a compact design. The primary electron beam is square wave modulated at 500 Hz. The use of phase-sensitive detection provides discrimination of the partial pressure signal from sputtering discharge induced light and black body light produced by the thoria—iridium filament. The electron beam current is set to 500 jzA with 100 V acceleration potential relative to ground. These values are a compromise between sensitivity (which would be enhanced at higher current levels) and a reduction in the tendency to form a discharge at the upper pressure ranges. An addi-
943
tional benefit of lower currents is the reduction in space charge induced non-linearities in the photon output and somewhat longer filament life. Figure 2 shows that the OGC was implemented as three subsystems connected by cables. The transducer subsystem contains the electron beam excitation source, thin film filters and PMTs. The sensor control unit subsystem contains current-to-voltage conversion, amplification, filtering, demodulation and multiplexing as well as sensor drive electronics and high voltage for the PMT dynode voltage. The electronic chassis subsystem consists of data entry and display, and analog-to-digital and digital-to-analog conversion of the signal and outputs respectively. An 8-bit microprocessor provides the process control phase sequence, control loop integral and differential functions and also linearizes the optical output with cross-sensitivity corrections at a rate of 4 Hz. The unit that has been used in this study has been configured to monitor two gases simultaneously. This is adequate to control a practical II
IASC
I
Ar.~lIN~I’I
N, Ar
SCU
Ar
ps
NaAr
CM
Ar Ar
N,
A
IV
ceEl
PLV
1 MFS
I
i~m
N
DD
2Ar
OC NT. Fig. 2. The OGC interfaced to a reactive sputtering system. The basic system consists of the transducer (T) described in Fig. 1 connected to the sensor control unit (SCU) which provides power and signal processing. The refined signals are sent to the operator’s console (OC) for display and subsequent control purposes. Information concerning the system pressure may be obtained from a capacitance manometer (CM) and gas flow from a mass flow sensor (MFS). The console is configured to control the cathode power supply (PS), to open or close the throttle valve (TV) and to regulate the flow of argon and nitrogen through piezoelectric leak valves (PLV). In this way the degree of compound formation on the cathode (C) is controlled so high metal arrival rates at the substrate (S) are balanced with the proper impingement rate of nitrogen to form the desired compound. Interfaces with other portions of the vacuum coating plant are through the relay interface (INT) and the computer bus (CB).
944 10.0
8.0
S C
.
.:~
c
60
M
~
—~
h
30 SEC
4.0
N
2 2.0
TIME
-~
Fig. 3. Plot of nitrogen mass flow in standard cubic centimeters per minute required to maintain a constant partial pressure during the production of TiN from a titanium cathode. The sputtering current was 4 A, N2 pressure was 9 x 10~ Torr and argon pressure was 3.0 x i0~ Torr. This plot was typical of a cathode that was not sufficiently presputtered prior to the inlet of reactive gas.
process situation such as the use of Ar—N2 or Ar—O2 to produce TiN or TiO2 respectively. The unit was further configured to provide information about the residual gas composition of the chamber prior to backfilling with the process gas. A motor and lead screw mechanism is used to position a second pair of optical filters in front of the PMTs. We have used this to monitor the system pumpdown of water vapor and carbon dioxide and can consequently make a judgment on the suitability of the vacuum prior to processing. In addition, monitoring the N2 line can give an indication of air leaks. The OGC was configured as a process controller. Once the optical output is calibrated using a capacitance manometer, the unit is directed to maintain a specific partial pressure of the gases. It executes a process sequence which includes a pre-deposition sputter phase that cleans the target prior to the admission of the reactive gas. A mass flow transducer was also interfaced with the OGC. This information was used to produce a time chart of the quantity of gas necessary to maintain a specific partial pressure. This information was sometimes revealing as shown in Fig. 3. The dramatic shift in flow required to maintain a constant arrival rate of reactive gas as the condition of the cathode changes with use should be noted. 4. Performance
The key performance areas of the OGC are two; sensitivity and stability. We have arbitrarily defined the minimum detectable limit as (S + N)/S =
945
TABLE 1 Minimum detectable limits of common gasesa Gas
Ar CO
Wavelength
4596
(A)
Minimum detectable limits (Torr) Above background
In 3 x iO~ TorrAr
4X105
—
2 2890 1 x 10~ 1 x 106 H20 3100 1 x 106 3 x 10~ 6 1.5x105 N2 3914 3 x iO~ 1 x 106 02 2594 5x1Cr aThese values are based on 500 .zA emission current and a measurement time of 0.25 s.
2. This means that the output S due to a just detectable partial pressure of a given gas is equal to the fluctuation N in the measurement due to noise. Table 1 shows our measured detectability in Torr for a variety of common gases for each gas by itself and also as a component in a 3 mTorr Ar background. We have found three sources of noise leading to these limits. At longer wavelengths a significant contribution from the hot filament was found. Gases that have a cross-talk component from argon have a significant fluctuation noise component added to the pertinent signal when measured in combination. Finally, for gases that are monitored at the shortest wavelengths (with the clearest signals) the noise has been measured to be less than a factor of 2 times larger than the theoretical fluctuation limit for the number of photons collected and converted. These tabulated values represent the performance of the prototype. Further improvements are possible; a minimum detectable limit of 2 x 10—6 Torr of oxygen has been achieved in a laboratory prototype. Although these results do not compare favorably with the sensitivity of a quadrupole mass spectrometer, the absence of pressure reduction compensates for the reduced sensitivity in many cases. In this way, even though the OGC has a lower sensitivity to N 2 than a quadrupole mass spectrometer, its overall signal-to-noise ratio is superior in a situation requiring pressure reduction. Stability is considered to be an important advantage of this method. The present authors have measured the stability over 30 days to be better than ±3.5%. The measurements were taken for both a normally encountered argon pressure (10 mTorr) and a low pressure of oxygen (10~Torr). The measurements were not found to suffer from systematic errors but seemingly random differences. It is felt that much of this is related to normal zero readjustments of the capacitance manometer that was used as a reference. In addition to the excellent stability, we found that filaments last on the order of a year in most gases. Lifetimes of a few hours have been experienced in chlorofluorocarbons and a few weeks to a few months in hydrocarbon mixtures.
946
AlOS
0 Se~r~ds
__.~._I
I~ lOll)
I
5
Fig. 4. OGC response to simulated cathode arcs during the production of TiN. The two traces are the partial pressures of argon and N 2 as measured and controlled by the OGC. The power supply was briefly (about 0.5 s) shut off during a run to show the reactive gas recovery3 capability. min~ compared The amount with 1.2 of nitrogen standard required cm3 min1 during to maintain sputtering thewas desired 8.5 standardpressure partial cm without sputtering.
The instrument has been used to produce TiN films in a wide range of colors as well as to reproduce the desired gold color over extended periods of time. The OGC has controlled piezo valves for both N 2 and argon, providing stable deposition conditions through changing target conditions as well as cathode arcs as shown in Fig. 4. The reactive deposition rate has been measured by a quartz crystal deposition monitor to be 80% of the metal rate. The high removal rate of metal compared with the removal rate possible with a nitrided cathode qualifies the process as “high rate” [3]. The establishment of high metal removal rates is important to the economic production of coatings. This equipment has also been used to produce reactively sputtered TiO2 films with good rate and no observable metal impurities. 5. Conclusion The OGC unit herein described has been demonstrated to be a practical method of reactive gas control for sputtering processes. It has demonstrated high stability over long periods of time and adequate sensitivity for processes of present commercial interest. The transducer is simple and reliable, demonstrating long life and simple, easy to maintain construction. In addition to the initial use as a process controller for reactive processes it appears to have value as a gas-selective vacuum gauge for many low vacuum process applications such as vacuum furnaces and commercial vacuum processing. This technology is easily expanded to handle more than two gases simultaneously through the use of more filters and PMTs. The major difficulty of this expansion is understanding the interactions between arbitrary gases and their effect on the photon flux.
947
The OGC appears to fill a unique niche for vacuum process control. It provides moderate performance gas analysis and control capabilities in a pressure range used by many important industrial processes. The unit’s stability and simplicity will make it convenient to use and maintain and therefore make partial pressure analysis a practical option in these process plants. References 1 2 3 4
C. A. Gogol, U.S. Patent 4,692,630, September 8, 1987. C. Lu, M. J. Lightner and C. A. Gogol, J. Vac. Sci. Technol., 14 (1976) 103. W. D. Sproul et al., U.S. Patent 4,428,811, 1984. E. U. Condon and G. H. Shortley, The Theory of Atomic Spectra, Cambridge University Press, London, 1979, pp. 109 - 114.
939
A NEW TYPE OF TRANSDUCER FOR PARTIAL PRESSURE CONTROL* C. A. GOGOL and R. MUELLER Leybold Inficon Inc., 6500 Fly Road, East Syracuse, NY 13057 (U.S.A.) (Received April 12, 1988)
Summary A new transducer for controlling the partial pressure of sputtering gases is described. Utilizing the electron impact emission spectroscopy (EIES) principle, an instrument capable of operating in a total pressure environment of 100 X iO~Torr or more with minimum detectable partial pressures to 10’ Torr is described. These capabilities seem ideal for controlling reactive deposition processes utilizing N2, 02 or hydrocarbons. Because of the high operating pressure capabilities, this transducer does not require the complication of high vacuum pressure reduction and thus eliminates a major source of unscheduled maintenance. The operating principles of EIES and the sensitivities that were determined for the various gases tested are described in this paper. The present authors have found the technique to have high stability over extended periods of time (about ±3.5% over 30 days) and sufficient sensitivity to control processes of present practical interest. In addition to the use for direct partial pressure control the technique has utility for monitoring common residual gases such as H20 and CO2. in an analyzer mode, as a precursor to starting the reactive process.
1. Introduction Over the past several years the present authors have been developing a new technology for partial pressure analysis and control. This technology, based on electron impact emission spectroscopy [1, 2] has a set of unique properties and advantages for the intended applications, e.g. partial pressure control of reactive high rate sputtering [3] and low vacuum gas analysis. Specifically, the advantages are perceived to be as follows. (A) Stability: a long-term constancy far exceeding that demonstrated by other instruments used today. (B) Simplicity: a design that requires no moving parts, no critical voltages and no close tolerances. These all combine to make for an instru*paper presented at the 15th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 11 - 15, 1988. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
940
ment that has inherent long life and is relatively free from deterioration due to common contamination. (C) High operating pressure: the present implementation allows pressures up to 100 X iO~ Torr to be directly monitored without pressure conversion. (D) Wide dynamic range: a practical, useful range of up to 6 orders of magnitude, covering the minimum sensitivity (about 1 X 10~Torr for some gases) to the maximum operating pressure. (E) Quick response: the combination of good signal-to-noise ratios and unrestricted mixing of gases from the chamber. (F) Ease of maintenance: because a pressure reduction system is not needed and the instrument utilizes simple structures and subsystems, maintenance needs are minimal and performed by technicians with general skills. (G) Specificity for a wide range of gases: with the simple exchange of thin film optical filters a wide range of common gases may be monitored or controlled. These include argon N2, H20, 02, CO, CO2 and some hydrocarbons such as C2H2 and CH4. With these key points in mind, the instrument’s operation, performance and application are described.
2. Transducer description The operation of this transducer is based on the energy exchange between a low current (500 ptA), medium voltage (100 V) primary electron beam and the gas molecules or atoms in the vacuum chamber. Gas molecules that receive energy from a close encounter with a primary electron relieve that energy by emitting a photon equal in energy to the originally transferred energy. Since the allowed transitions are energy discrete, based on the electron configuration of the specific species, the photons produced are therefore frequency discrete and attributable to a specific gas species. Most gases have several transitions that are excited with high probability and will have a signature of wavelengths that allow unique identification even when the preferred transitions of several species occur very close in frequency. Together with the ability to identify a gas species by its photon frequency signature the ability to deduce its density is available through the measurement3)ofdue the tophoton flux. In simplistic terms, from the total the specific electron transition statephoton i to k flux J,,. (s’ cm number density N (cm3) of a specific gas species by the is related to the following:
=N(i/e)uk~
(1) where i (A/cm2) is the primary electron current, e (A s) the coulombic charge and Uki (cm2) the cross-section for exciting the gas atom from state k to i with the specific constant energy of the primary electron. The implica~ik
941
tion of this simple relationship is that, if the primary electron voltage and current are held constant, the total number of photons produced is directly related to the number density of the species. Thus at constant temperature the photon flux at a specific wavelength may be thought to be directly proportional to the partial pressure of a particular gas species. For this reason, we have chosen to call this instrument an optical gas controller (OGC). Of course it is not quite so simple in practice. While eqn. (1) might hold for the case of a very dilute gas, operation at higher pressure means that the effects of pressure broadening and self-absorption will be noticeable for many gases [4]. In addition, the general environment may consist of several gases simultaneously. At the pressures encountered, especially near the upper operating range, chemical interactions between the gases and, perhaps more importantly, multiple collisions of the primary electrons with gas molecules may significantly alter the output. Nevertheless, all the studied cases are at least monotonic with pressure. In the cases of primary interest, algorithms have been developed that take the chemical and pressure effects into account in a sufficiently accurate manner. These algorithms are series expansions, based on experimental observation. Once photons are produced, a portion passes through a vacuum-sealed window and is then frequency selected by a thin film filter. These “species”specific photons are converted to an electric current by a compact photomultiplier tube (PMT). Subsequent current-to-voltage conversion, demodulation and amplification once properly calibrated and linearized produce a display indication of partial pressure p as follows: PK(~Ta,Nb,,1’)k’(hik
(2)
where K(Na, Nb, P) is a normalization function of the densities Na, Nb, of the various species and total pressure P. k’ is a gauge factor that takes into account all the various photon collection and conversion efficiencies and is established at the time of calibration. The normalization function is used to provide reasonably accurate quantitative results for a specific situation. Qualitative information and trend analysis of gases is of course possible without knowledge of the normalization function. ...,
3. Implementation The transducer described above has been implemented in hardware as shown in Fig. 1. The basic exciting unit is built around the standard 2.75 in metal seal sexless vacuum flange. In this way the gases from the chamber have direct contact with the excitation volume. The 1.375 in opening offers minimum impedance to the diffusion flow of the gases to the excitation volume. As a consequence, delays and selectivities caused by a sampling system are avoided.
942
PMT
LS
:: ~
OF
/
M
__________
~-
______
Fig. 1. The basic OGC transducer. An electron beam (EB) from the hot filament (F) of the emitter (E) passes through the excitation volume (EV) where energy is exchanged
with gas molecules. Subsequent de-excitation produces photons (P) specific in wavelength for each species. A portion of the photons pass through the vacuum-sealed quartz window (W) through a bandpass optical filter (OF) and are converted into an electrical current in a photomultiplier (PMT). A filter and photomultiplier combination is used for each channel. Other features of the transducer are a thermal shield (TS) for fast warm-ups, a Faraday trap (FT) for the suppression of secondary electrons and a light shield (LS) to reduce unwanted radiation at the PMTs. The transducer is built around a standard 2.75 in vacuum flange (VF) and has provision for an auxiliary pump port (PP) for further pressure reduction. Filters can be exchanged by powering a lead screw with motor (M).
The OGC is a density-sensitive transducer. Because the apparent gas density can change as a function of temperature, a low thermal mass shield is introduced to reduce the time it takes for the filament to warm the structures that come in contact with gas. In this manner even batch processing situations can be handled without temperature-induced sensitivity drift. Hammatsu R1414 PMTs were used; they provide adequate signal gain and also a compact design. The primary electron beam is square wave modulated at 500 Hz. The use of phase-sensitive detection provides discrimination of the partial pressure signal from sputtering discharge induced light and black body light produced by the thoria—iridium filament. The electron beam current is set to 500 jzA with 100 V acceleration potential relative to ground. These values are a compromise between sensitivity (which would be enhanced at higher current levels) and a reduction in the tendency to form a discharge at the upper pressure ranges. An addi-
943
tional benefit of lower currents is the reduction in space charge induced non-linearities in the photon output and somewhat longer filament life. Figure 2 shows that the OGC was implemented as three subsystems connected by cables. The transducer subsystem contains the electron beam excitation source, thin film filters and PMTs. The sensor control unit subsystem contains current-to-voltage conversion, amplification, filtering, demodulation and multiplexing as well as sensor drive electronics and high voltage for the PMT dynode voltage. The electronic chassis subsystem consists of data entry and display, and analog-to-digital and digital-to-analog conversion of the signal and outputs respectively. An 8-bit microprocessor provides the process control phase sequence, control loop integral and differential functions and also linearizes the optical output with cross-sensitivity corrections at a rate of 4 Hz. The unit that has been used in this study has been configured to monitor two gases simultaneously. This is adequate to control a practical II
IASC
I
Ar.~lIN~I’I
N, Ar
SCU
Ar
ps
NaAr
CM
Ar Ar
N,
A
IV
ceEl
PLV
1 MFS
I
i~m
N
DD
2Ar
OC NT. Fig. 2. The OGC interfaced to a reactive sputtering system. The basic system consists of the transducer (T) described in Fig. 1 connected to the sensor control unit (SCU) which provides power and signal processing. The refined signals are sent to the operator’s console (OC) for display and subsequent control purposes. Information concerning the system pressure may be obtained from a capacitance manometer (CM) and gas flow from a mass flow sensor (MFS). The console is configured to control the cathode power supply (PS), to open or close the throttle valve (TV) and to regulate the flow of argon and nitrogen through piezoelectric leak valves (PLV). In this way the degree of compound formation on the cathode (C) is controlled so high metal arrival rates at the substrate (S) are balanced with the proper impingement rate of nitrogen to form the desired compound. Interfaces with other portions of the vacuum coating plant are through the relay interface (INT) and the computer bus (CB).
944 10.0
8.0
S C
.
.:~
c
60
M
~
—~
h
30 SEC
4.0
N
2 2.0
TIME
-~
Fig. 3. Plot of nitrogen mass flow in standard cubic centimeters per minute required to maintain a constant partial pressure during the production of TiN from a titanium cathode. The sputtering current was 4 A, N2 pressure was 9 x 10~ Torr and argon pressure was 3.0 x i0~ Torr. This plot was typical of a cathode that was not sufficiently presputtered prior to the inlet of reactive gas.
process situation such as the use of Ar—N2 or Ar—O2 to produce TiN or TiO2 respectively. The unit was further configured to provide information about the residual gas composition of the chamber prior to backfilling with the process gas. A motor and lead screw mechanism is used to position a second pair of optical filters in front of the PMTs. We have used this to monitor the system pumpdown of water vapor and carbon dioxide and can consequently make a judgment on the suitability of the vacuum prior to processing. In addition, monitoring the N2 line can give an indication of air leaks. The OGC was configured as a process controller. Once the optical output is calibrated using a capacitance manometer, the unit is directed to maintain a specific partial pressure of the gases. It executes a process sequence which includes a pre-deposition sputter phase that cleans the target prior to the admission of the reactive gas. A mass flow transducer was also interfaced with the OGC. This information was used to produce a time chart of the quantity of gas necessary to maintain a specific partial pressure. This information was sometimes revealing as shown in Fig. 3. The dramatic shift in flow required to maintain a constant arrival rate of reactive gas as the condition of the cathode changes with use should be noted. 4. Performance
The key performance areas of the OGC are two; sensitivity and stability. We have arbitrarily defined the minimum detectable limit as (S + N)/S =
945
TABLE 1 Minimum detectable limits of common gasesa Gas
Ar CO
Wavelength
4596
(A)
Minimum detectable limits (Torr) Above background
In 3 x iO~ TorrAr
4X105
—
2 2890 1 x 10~ 1 x 106 H20 3100 1 x 106 3 x 10~ 6 1.5x105 N2 3914 3 x iO~ 1 x 106 02 2594 5x1Cr aThese values are based on 500 .zA emission current and a measurement time of 0.25 s.
2. This means that the output S due to a just detectable partial pressure of a given gas is equal to the fluctuation N in the measurement due to noise. Table 1 shows our measured detectability in Torr for a variety of common gases for each gas by itself and also as a component in a 3 mTorr Ar background. We have found three sources of noise leading to these limits. At longer wavelengths a significant contribution from the hot filament was found. Gases that have a cross-talk component from argon have a significant fluctuation noise component added to the pertinent signal when measured in combination. Finally, for gases that are monitored at the shortest wavelengths (with the clearest signals) the noise has been measured to be less than a factor of 2 times larger than the theoretical fluctuation limit for the number of photons collected and converted. These tabulated values represent the performance of the prototype. Further improvements are possible; a minimum detectable limit of 2 x 10—6 Torr of oxygen has been achieved in a laboratory prototype. Although these results do not compare favorably with the sensitivity of a quadrupole mass spectrometer, the absence of pressure reduction compensates for the reduced sensitivity in many cases. In this way, even though the OGC has a lower sensitivity to N 2 than a quadrupole mass spectrometer, its overall signal-to-noise ratio is superior in a situation requiring pressure reduction. Stability is considered to be an important advantage of this method. The present authors have measured the stability over 30 days to be better than ±3.5%. The measurements were taken for both a normally encountered argon pressure (10 mTorr) and a low pressure of oxygen (10~Torr). The measurements were not found to suffer from systematic errors but seemingly random differences. It is felt that much of this is related to normal zero readjustments of the capacitance manometer that was used as a reference. In addition to the excellent stability, we found that filaments last on the order of a year in most gases. Lifetimes of a few hours have been experienced in chlorofluorocarbons and a few weeks to a few months in hydrocarbon mixtures.
946
AlOS
0 Se~r~ds
__.~._I
I~ lOll)
I
5
Fig. 4. OGC response to simulated cathode arcs during the production of TiN. The two traces are the partial pressures of argon and N 2 as measured and controlled by the OGC. The power supply was briefly (about 0.5 s) shut off during a run to show the reactive gas recovery3 capability. min~ compared The amount with 1.2 of nitrogen standard required cm3 min1 during to maintain sputtering thewas desired 8.5 standardpressure partial cm without sputtering.
The instrument has been used to produce TiN films in a wide range of colors as well as to reproduce the desired gold color over extended periods of time. The OGC has controlled piezo valves for both N 2 and argon, providing stable deposition conditions through changing target conditions as well as cathode arcs as shown in Fig. 4. The reactive deposition rate has been measured by a quartz crystal deposition monitor to be 80% of the metal rate. The high removal rate of metal compared with the removal rate possible with a nitrided cathode qualifies the process as “high rate” [3]. The establishment of high metal removal rates is important to the economic production of coatings. This equipment has also been used to produce reactively sputtered TiO2 films with good rate and no observable metal impurities. 5. Conclusion The OGC unit herein described has been demonstrated to be a practical method of reactive gas control for sputtering processes. It has demonstrated high stability over long periods of time and adequate sensitivity for processes of present commercial interest. The transducer is simple and reliable, demonstrating long life and simple, easy to maintain construction. In addition to the initial use as a process controller for reactive processes it appears to have value as a gas-selective vacuum gauge for many low vacuum process applications such as vacuum furnaces and commercial vacuum processing. This technology is easily expanded to handle more than two gases simultaneously through the use of more filters and PMTs. The major difficulty of this expansion is understanding the interactions between arbitrary gases and their effect on the photon flux.
947
The OGC appears to fill a unique niche for vacuum process control. It provides moderate performance gas analysis and control capabilities in a pressure range used by many important industrial processes. The unit’s stability and simplicity will make it convenient to use and maintain and therefore make partial pressure analysis a practical option in these process plants. References 1 2 3 4
C. A. Gogol, U.S. Patent 4,692,630, September 8, 1987. C. Lu, M. J. Lightner and C. A. Gogol, J. Vac. Sci. Technol., 14 (1976) 103. W. D. Sproul et al., U.S. Patent 4,428,811, 1984. E. U. Condon and G. H. Shortley, The Theory of Atomic Spectra, Cambridge University Press, London, 1979, pp. 109 - 114.