Patent Number: 
Section: 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 xe2x80x9cdirty,xe2x80x9d 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"" LabView(trademark), 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 xe2x80x9cdriftxe2x80x9d 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 xe2x80x9ctruexe2x80x9d 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.