Abstract:
Method and apparatus for rapid monitoring of pressure in a chamber equipped with a mass spectrometer by using the primary beam current in conjunction with a conventional pressure gauge. The conventional gauge allows frequent calibration of the relationship of the beam current to the chamber pressure, preventing excessive drift in the system. An advantage of the system is that it takes advantage of instruments already present in a typical spectrometry apparatus.

Description:
FIELD OF THE INVENTION 
     The present invention relates to pressure measurement in conjunction with mass spectrometry, and in particular to continuous pressure measurement by monitoring of ion beam current. 
     BACKGROUND OF THE INVENTION 
     Mass spectrometry is now widely used in a variety of fields. In particular, it is used to monitor and control a number of processes in semiconductor fabrication, including chemical and physical vapor deposition and ion implantation. Accurate measurement and control of fabrication processes is essential in semiconductor manufacture. One important parameter to be measured in many processes is the total chamber pressure. 
     Pressure in vacuum chambers can be measured using a variety of devices, including thermocouple gauges, ionization gauges, rotating-disc viscosity gauges, and others. The type of gauge used in any particular system is generally selected according to considerations of cost, desired accuracy, and expected pressure range. Typically, pressures in the millitorr range are measured with a thermocouple gauge or other gauge that measures heat dissipation, whose readout times depend on the mass of the heated element and the heat transfer coefficient. These gauges cannot practically deliver pressure data at rates faster than a few Hz. Even commercial ultra low pressure ion beam gauges provide discrete readouts at only moderate frequencies, e.g., about 1 Hz. 
     Semiconductor manufacturing lines currently do not dynamically use pressure data to control deposition rates or other process parameters. While pressure may be monitored during the fabrication process, the low data rates of the pressure gauges described above make it infeasible to use them for process control. 
     Much faster pressure readings can be obtained by monitoring the primary ion beam current in the collector of a mass spectrometer. The ion beam current is related to the ion source pressure, which can in turn be related to chamber pressure. Such a system is described, for example, in U.S. Pat. No. 5,834,770 to Holkeboer et al. Unfortunately, pressure readings obtained in this way are often not particularly stable. “Drift” in the relationship between beam current and ion source pressure can occur as the source parameters are varied (e.g., to tune the instrument), as the source becomes dirty, as the source filament ages, or for other reasons. Thus, primary beam current is rarely used to monitor pressure in these systems. 
     A need therefore still exists for a system which can provide fast (preferably continuous), accurate pressure measurement in mass spectrometry systems. 
     SUMMARY OF THE INVENTION 
     The present invention allows rapid monitoring of pressure in a chamber equipped with a mass spectrometer by using the primary beam current in conjunction with a conventional pressure gauge. The conventional gauge allows frequent calibration of the relationship of the beam current to the chamber pressure, preventing excessive drift in the system. An advantage of the system is that it takes advantage of instruments already present in a typical spectrometry apparatus. 
     In one aspect, the invention comprises apparatus for monitoring chamber pressure, including an ion source which produces an ion beam, a collector where beam current can be continuously measured, and a pressure gauge which can be used to obtain a series of discrete pressure measurements (e.g., at a rate of less than about 100 Hz, or at a rate of less than about 1 Hz). The apparatus further comprises analysis means which combine the beam current measurement and the pressure gauge measurements to continuously determine the chamber pressure, for example by using the beam current to determine an offset to the most recently measured gauge pressure. The apparatus may comprise orthogonal acceleration means that allow the ion beam to be used in time-of-flight mass spectrometry, or the ion beam may be a component of another type of mass spectrometer. 
     In another aspect, the invention comprises methods of monitoring chamber pressure in a chamber equipped with a pressure gauge, an ion source that produces an ion beam, and an ion collector from which a beam current can be measured. The methods include monitoring the pressure gauge to obtain a series of discrete pressure measurements (e.g., at a rate of less than about 100 Hz, or at a rate of less than about 1 Hz), and continuously monitoring the beam current (e.g., at a rate of at least about 1 kHz, 10 kHz, or 50 kHz). The continuous beam current measurement and the discrete pressure measurements are then combined to yield a continuous chamber pressure measurement. The continuous pressure measurement may serve as an input to an automated process control system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is described with reference to the several figures of the drawing, in which, 
     FIG. 1 is a schematic diagram of a time-of-flight mass spectrometer; and 
     FIG. 2 is a graph showing absolute pressure and measured pressure as a function of time. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a time-of-flight mass spectrometer according to one embodiment of the invention. The ion source  10  emits a primary ion beam  12 , which is focused and/or collimated. An orthogonal accelerator  14  applies carefully timed voltage pulses to deflect a fraction of the ions  20  into a secondary collector  22  for mass analysis. The remainder of the ions continue undeflected to the primary collector  16 . The current due to these ions striking the collector  16  is measured at ammeter  18  to determine the beam current (the ammeter  18  may be an electrometer which amplifies small currents into measurable voltages). A separate conventional pressure gauge  24  also measures the pressure in the chamber; the ammeter  18  and the pressure gauge  24  are connected to a processor  26 . 
     When the fraction of ions being deflected to the secondary collector  22  is known, the current at the primary collector  16  can be corrected to determine the ion source pressure (the number of ions leaving the ion source  10 ). The ion source pressure is a function of the chamber pressure (among other parameters). Thus, the beam current can be used to determine the pressure in the chamber. Beam current can be read at rates as fast as 50 kHz, effectively continuously. (In this disclosure, measurement rates greater than about 1 kHz are treated as continuous). 
     As discussed above, beam current alone is not generally used to monitor pressure, because it can vary with changes in instrument tuning, as the ion source becomes “dirty,” as a source filament ages, or in response to other perturbations of the spectrometry system. All of these changes typically take place over a timescale of minutes or even hours, however. According to the invention, the effects of these perturbations can be greatly reduced or even eliminated by calibrating the relationship between beam current and pressure by reference to the conventional pressure gauge on a frequent basis. 
     In preferred embodiments, the ammeter  18  and the pressure gauge  24  are connected to analog-to-digital converters (not shown) to produce digital signals. These signals are combined in a data processor  26 , which generates a combined signal that may be used for process control as described below. The data processor may comprise a computer running standard data capture software, such as National Instruments&#39; LabView™, or it may be a custom processor. Alternatively, the analog-to-digital converters may be omitted, and the processor  26  may be an electrical circuit used to combine the ammeter and pressure gauge signals to produce an analog output. 
     In preferred embodiments of the invention, the relationship between beam current and pressure is recalibrated at the reading frequency of the pressure gauge. This relationship is shown schematically in the graph of FIG.  2 . Solid line  30  represents the actual chamber pressure as a function of time. This pressure is measured by the pressure gauge at points  32 . The pressure at times between the pressure gauge measurements is determined by reference to the beam current, as shown by dotted segments  34 . These measurements may deviate from the true pressure as shown, as “drift” in pressure measurement occurs (deviations have been exaggerated in FIG. 2 for clarity). At each gauge measurement, the calibration of the beam current/pressure relationship is reset, and changes in pressure from the newly determined gauge pressure are calculated using the beam current. 
     In other embodiments of the invention, less frequent pressure gauge measurements may be used. For example, the “true” pressure may be measured once a minute or even less frequently, as long as the interval is short compared to the timescale of drift of the beam current/pressure relationship. 
     Comparison of the pressure gauge and beam current measurements to produce the curve of FIG. 2 may be performed by standard circuits familiar to those skilled in the art, or by computer-based measurement and acquisition systems. These measurements may then be directly displayed on a monitor or other output means, or may be used to provide feedback for process control, for example for concentration monitoring in deposition systems. Even if a dynamic feedback system is not used, the pressure measurement system of the invention can be used to very quickly cut off a plasma torch during deposition in response to a pressure fluctuation outside normal tolerances. Such a cut-off system reduces the risk of destroying a wafer which may be worth many thousands of dollars. 
     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification be considered as exemplary only, with the true scope of the invention being indicated by the following claims.